More than a year after his conviction for operating a motor vehicle while under the influence of alcohol, the

*762defendant, John W. Bresnahan, moved for postverdict inquiry of the jurors. The trial judge had recused himself, and a different judge granted the defendant’s motion for postverdict inquiry. After holding evidentiary hearings, the judge allowed the defendant’s motion for a new trial on the basis that the jury had been exposed to an extraneous influence during deliberations and the Commonwealth had failed to prove that the incident did not prejudice the defendant. The Appeals Court vacated the order for a new trial and remanded the case for further inquiry into the bases for the defendant’s request for a postverdict inquiry of the jurors and, in particular, the role played by the defendant and his friend in contacting a juror, in apparent violation of the principles announced by this court in Commonwealth v. Fidler, 377 Mass. 192 (1979) (Fidler). See Commonwealth v. Bresnahan, 79 Mass. App. Ct. 353, 364 (2011). We granted the defendant’s application for further appellate review. We agree that remand for further inquiry is appropriate, but take this opportunity to clarify the procedure for conducting postverdict hearings where, although a defendant claims that the jury were exposed to an extraneous influence, there is also evidence that the defendant or someone acting on the defendant’s behalf may have violated the Fidler principles.

1. Background.1 a. Trial. On December 5, 2006, the defendant was tried in the District Court on a charge of operating a motor vehicle while under the influence of alcohol. The Commonwealth presented the testimony of two police officers. The first officer testified as follows. On November 7, 2005, at approximately 9 p.m., the officer was driving in the opposite direction from the defendant on a narrow street in Nantucket. The defendant was driving on the wrong side of the street, but after a moment “jerked” his car back to the correct lane. After stopping the defendant, the officer smelled a strong odor of alcohol on the defendant’s breath and observed an open beer can on the driver’s side floor and two empty beer cans on the front pas*763senger seat. Although the defendant stated that he had had only one beer that night, he was unsteady on his feet and failed multiple field sobriety tests.2 The second officer, who was called to the scene for backup and to watch traffic, testified that he saw the defendant sway from side to side and smelled a strong odor of alcohol on his breath. Both officers opined that the defendant was drunk.

The defendant testified that at the time of the incident, he suffered from late-stage Lyme disease, causing his joints to be sore and affecting his walking and bending, and from shingles, which caused severe pain in his back, shoulder, groin, and thighs. He submitted progress notes from his doctor to confirm that he had been treated for these conditions in 2004 and 2005. The defendant testified that not only was it physically difficult for him to complete the sobriety tests due to his medical conditions, but he was distracted by the lights of passing traffic and “bewildered” by the situation.

The jury began deliberating at 4:20 p.m. At 5:20 p.m., according to the trial judge’s notes, the jury sought an answer to a question; however, neither the trial transcript nor any other part of the record indicates what the jury’s question was or that they were brought back into the court room for an answer. Thirty-five minutes later, at 5:55 p.m., the jury returned a verdict of guilty. The judge sentenced the defendant to two and one-half years in a house of correction.

b. Motion for postverdict juror inquiry. One year after the trial, the defendant’s friend and former landlord, Barbara Constantine, contacted the defendant’s appellate counsel, by telephone. According to counsel’s affidavit submitted by the defendant in support of his motion for postverdict juror inquiry, Constantine contacted appellate counsel to tell him that a juror who served on the defendant’s trial (Juror A) had told Constantine that she, the juror, “was upset about the way that deliberations in the case had gone and wanted to talk to someone about it.” One week later, Constantine telephoned appellate counsel again to tell him that Juror A had provided a telephone number so that appellate counsel could contact her. He thereafter spoke *764with Juror A on the telephone twice. During the second conversation,3 Juror A stated that the trial judge had entered the jury room during deliberations and told the jurors, “I understand that you are having trouble reaching a verdict”; “You are not leaving here until you have reached a verdict”; and that the jury should “go with the group consensus.”

Appellate counsel moved to withdraw from the case, and the defendant’s present counsel became successor appellate counsel. The defendant filed a motion for postverdict juror inquiry, supported, as previously noted, by appellate counsel’s affidavit. No affidavit from either Constantine or Juror A was filed or offered. The trial judge recused himself from the case, and a different District Court judge denied the motion for lack of credible evidence because Juror A did not appear personally or submit her own affidavit. The defendant then filed a motion for reconsideration, supported by an additional affidavit from appellate counsel explaining that counsel had not solicited an affidavit from Juror A because he did not want to communicate with the juror again without the judge’s permission. This second judge vacated his previous order denying the motion and recused himself from further proceedings. A third District Court judge (motion judge) then was assigned to hear the matter.

After a hearing on June 10, 2008, the motion judge allowed the defendant’s motion for postverdict juror inquiry. On the basis of what Juror A told the defendant’s appellate counsel, as set out in counsel’s affidavit, the motion judge found the defendant had presented a “colorable claim” that there had been an improper, extraneous influence on the jurors during deliberations in the form of an ex parte communication between the trial judge and the jurors.

c. First evidentiary hearing. The motion judge held an evi-dentiary hearing on June 25, 2008, at the District Court in Nantucket. Four of the six jurors who had deliberated on the defendant’s case testified. Juror A testified to the following. The trial judge entered the jury room twice during the jury’s deliberations. The first time, the judge explained the procedure to be followed if the jury had any questions. On this first occa*765sion, a court officer opened the door for the judge, but Juror A could not remember if the court officer also came into the jury room. The second time, near the end of deliberations, the judge responded to the jury’s question about what to do if they could not agree on a verdict. The trial judge told the jury, “You have to agree .... [Ljisten to the judgment of your peers and agree.” Juror A also thought that the judge said something to the effect of, “[Y]ou are not leaving here until you have reached a verdict.” This time, the judge was “escorted in” by a court officer, but Juror A did not believe that the court officer remained in the room. The judge never indicated which way — guilty or not guilty — the jury should decide the case.

Juror A further testified that she spoke with Constantine three times, and Constantine had initiated all the conversations. The first time Constantine telephoned Juror A, probably in the spring of 2007, she asked Juror A why the jury took so long to reach their verdict, and Juror A replied, “I’m not sure I’m supposed to answer that.” Nonetheless, Constantine telephoned Juror A again, some months later, and Juror A responded to Constantine that she had nothing new to report. More recently — four or five months before the June 25, 2008, hearing — Constantine telephoned Juror A a third time and told her, “Well, you read in the paper all the time about what jurors have said about various famous trials,” and that it would be “legal” for Juror A to speak with her about the deliberations. During this conversation, Juror A decided to tell Constantine what the jury had discussed concerning the evidence in the case. Juror A also recalled that she “probably” talked to Constantine about the trial judge, and that she “probably” told Constantine the same account to which she had just testified.

A second juror (Juror B) testified that the trial judge never entered the jury room and never told the jurors that they had to reach a verdict. A third juror (Juror C) testified that the trial judge entered the jury room shortly after the jury had begun to deliberate to explain the process. She did not remember whether the judge was accompanied by anyone. According to Juror C, another juror asked what would happen if they were unable to reach a verdict by the end of the day, and the trial judge replied, “Well, we’d want you to stay until you made a decision.” He *766did not say that jurors should “go with the consensus” if the jury were unable to reach a verdict. Juror C also testified that the jury never indicated they were having difficulty deciding on a verdict.

A fourth juror (Juror D) testified that the trial judge had entered the jury room, but she was not sure if one of the court officers also had entered. Juror D remembered two conversations with the judge. In the first, which occurred about fifteen to thirty minutes after the jury had been deliberating, one of the jurors asked what would happen if they did not reach a decision, and the trial judge answered that the jury should come to a conclusion. The jury reached their verdict approximately fifteen to thirty minutes later. The second conversation occurred when the judge asked the jury to wait after they had returned to the court room and delivered the verdict. He then returned to the jury room and thanked the jurors for their time.

After the June 25 hearing, at the motion judge’s invitation, the trial judge submitted a written report of his memories of the trial. In it, the trial judge wrote that he was informed the jury had a question at 5:20 p.m., but then was told shortly thereafter that the jury had resolved the question on their own. He also stated that he never entered the jury room while the jury were deliberating, although, as was his custom, he did speak to the jurors in the jury room after he had taken the verdict in the court room and before he had sentenced the defendant. While he was speaking to the jurors after they had reached their verdict, one of the jurors asked what would have happened if the jury had not been able to reach a verdict, and the judge responded by paraphrasing the Tuey-Rodriquez charge.4

d. Ruling on motion for a new trial. Following the June 25 evidentiary hearing, the defendant filed a motion for a new trial on July 17, 2008. On August 7, the motion judge issued his decision on the motion, in which he found as follows. After beginning deliberations, the jury indicated that they were seeking information from the trial judge. In response, the trial judge *767entered the jury room without the knowledge of the prosecutor or defense counsel. While there, the judge did not tell the jurors that they must agree on a verdict or that they could not leave until a verdict was reached.5 However, the trial judge did tell the jury that they were to come to a decision, “without any qualifying language concerning what would happen if they did not or what they should do if an impasse was reached.”

The motion judge found that the defendant had proved by a preponderance of the evidence that the jury were exposed to an “extraneous matter,” meaning the trial judge’s ex parte remarks to them during deliberations. The motion judge accordingly determined that the burden shifted to the Commonwealth to prove beyond a reasonable doubt that the defendant was not prejudiced by the extraneous matter. See Fidler, 377 Mass. at 201. He concluded that the Commonwealth had failed to prove the absence of prejudice because “[t]he jurors were clearly given the impression that their options were to find the defendant guilty or find him not guilty . . . [and] that advising the court that they were deadlocked was not an option.” He also found that the Commonwealth’s evidence against the defendant, while substantial, was not overwhelming, and that the jury might not have convicted the defendant if not for the misleading ex parte instructions. Because the Commonwealth failed to prove the absence of prejudice beyond a reasonable doubt, the motion judge allowed the defendant’s motion for a new trial.

e. Second and third evidentiary hearings. In response to a motion for reconsideration filed by the Commonwealth, the motion judge held a second evidentiary hearing on November 20, 2008, during which he heard the testimony of a fifth juror, the trial judge, the defendant, the trial prosecutor, and three court officers. The fifth juror (Juror E) testified that the jury had a question and that they received an answer outside the court room that was not in writing. The juror recalled that either the *768trial judge gave the answer or the jury were told that they already had been given an answer. Juror E further stated that he did not recall that the trial judge had helped the jury resolve any issue about the meaning of “reasonable doubt.” The motion judge inferred from this somewhat vague testimony that the jury likely received the answer to their question in the jury room, because they did not get an answer in writing or in the court room; the motion judge therefore found Juror E’s testimony supportive of the testimony of Jurors A, C, and D that the judge had indeed provided ex parte instructions to the jury in the jury room.

The trial judge testified consistently with his prior written report. The trial prosecutor and the court officers testified that they never saw the trial judge enter the jury room.

On December 16, 2008, a third and final evidentiary hearing was held at which the chief court officer testified.6 He stated that he was in the lobby with the trial judge for the entire time the jury were deliberating, and the trial judge never left. He further testified that during deliberations, a female court officer knocked on the door of the lobby and told the trial judge that the jury had a question. The trial judge asked the officer to have the question put in writing and called the attorneys into his lobby, where they waited for thirty to forty-five minutes. When the court officer returned, she stated that the jury had reached a verdict.

f. Ruling on motion for reconsideration. After hearing the testimony at the second and third hearings, the motion judge continued to credit the accounts of the jurors who had testified at the original evidentiary hearing to the effect that the judge had delivered ex parte remarks during deliberations. The motion judge “in large part . . . rejected] evidence to the contrary presented at the November and December hearings.” Therefore, he let stand his previous order granting a new trial.

g. Appeal. In its decision on the Commonwealth’s appeal from the motion judge’s order, the Appeals Court concluded that the judge erred by “engaging] in a postverdict examination of jurors prematurely and without affording the Commonwealth the opportunity to explore whether the defendant’s *769claim of an extraneous jury influence was orchestrated by a surrogate acting on behalf of the defendant.” Commonwealth v. Bresnahan, 79 Mass. App. Ct. at 353. The court suggested that proof of such orchestration by the defendant through his friend Constantine might have led the motion judge in the first instance to decline any postverdict inquiry of the jurors, and in any event was likely to have “colored the judge’s over-all credibility assessments and factual determinations.” Id. at 364. The Appeals Court therefore vacated the order granting the defendant’s motion for a new trial and remanded the case to a different District Court judge for additional inquiry into “the basis underlying the defendant’s showing in support of his request for posttrial contact of the jurors, including, without limitation, examination of Barbara Constantine.” Id. For reasons somewhat different from the Appeals Court, we vacate the order granting the defendant’s motion for a new trial and remand the case to the same motion judge for additional inquiry of Constantine and the defendant.

2. Discussion, a. Claim of Fidler violation by or on behalf of the defendant. This case implicates several significant interests: on the one hand, a defendant’s right to trial by an impartial jury, guaranteed by the Sixth Amendment to the United States Constitution and art. 12 of the Massachusetts Declaration of Rights; and on the other, the finality of jury verdicts and protection of jurors from unwelcome solicitation or harassment by litigants following their jury service. See Fidler, 377 Mass. at 196-197; Woodward v. Leavitt, 107 Mass. 453, 460-461 (1871). See also Diehm, Impeachment of Jury Verdicts: Tanner v. United States and Beyond, 65 St. John’s L. Rev. 389, 394-406 (1991); Carlson & Sumberg, Attacking Jury Verdicts: Paradigms for Rule Revision, 1977 Ariz. St. L.J. 247, 250-251 (Carlson & Sumberg).

This court attempted to balance these concerns in Fidler, supra at 204, by prohibiting counsel, litigants, and those acting for them from any postverdict contact with jurors absent permission from a judge.7 The Fidler proscriptions in relevant part have been adopted as a rule of professional conduct. See Mass.

*770R. Prof. C. 3.5 (d), 426 Mass. 1391 (1998) (rule 3.5 [d]). The rule bars attorneys from initiating contact with jurors after a verdict is rendered without court approval, although attorneys may respond to juror-initiated communications so long as such response is not intended to harass, embarrass, or influence future jury service.8

Against this backdrop, the Commonwealth claims that the motion judge abused his discretion by refusing to investigate the circumstances underlying Constantine’s and appellate counsel’s conversations with Juror A before granting the defendant’s motion for postverdict juror inquiry at an evidentiary hearing. It theorizes that if Constantine acted on the defendant’s behalf in contacting Juror A, any information obtained through that contact cannot be used to impeach the jury’s verdict.9 In essence, the Commonwealth seeks to institute an exclusionary *771rule that would prevent a judge from hearing or at least considering evidence about an alleged extraneous influence on a jury if the initial information was gained through improper juror contact. The claim for such a rule is premised on the Commonwealth’s reading of the following sentence in Fidler: “Any counsel or litigant who independently contacts jurors ‘acts at his peril, lest he be held as acting in obstruction of the administration of justice ....’” Fidler, 377 Mass. at 204, quoting Rakes v. United States, 169 F.2d 739, 745-746 (4th Cir.), cert. denied, 335 U.S. 826 (1948).10 We disagree: there is nothing in the quoted sentence or in Fidler generally to indicate that evidence obtained in violation of the Fidler principles is automatically subject to an exclusionary rule. See Commonwealth v. Solis, 407 Mass. 398, 401-402 (1990) (rejecting imposition of exclusionary rule where defendant’s counsel solicited information from jurors after trial while waiting near public elevators in court house, in violation of Fidler principles, but defendant himself was not involved).

In any event, we find no justification for adoption of a strict exclusionary rule whenever a violation of the Fidler principles is found to have occurred. Where a defendant’s Sixth Amendment and art. 12 right to a fair trial is at stake, a court cannot ignore credible evidence that a trial judge entered the jury room during deliberations. Cf. Parker v. Gladden, 385 U.S. 363, 363-365 (1966) (per curiam) (granting defendant new trial because of bailiff’s statements to deliberating jurors regarding defendant’s guilt); id. at 366-367 (Harlan, J., dissenting) (describing defendant’s extensive solicitation of jurors for evidence to support new trial motion). See Carlson & Sumberg, supra at

*772269 (“While courts may choose to ignore reports of jury misconduct in other situations, this posture is unacceptable in criminal cases. Where the misconduct deprives a defendant of his [Sjixth [Ajmendment rights, courts must consider the misconduct 11

Our refusal to adopt a strict exclusionary rule, however, does not mean that Constantine’s conduct was proper or, more particularly, that the motion judge should have declined to investigate the Commonwealth’s claim that Constantine may have initiated contact with one or more jurors.12 As suggested in Commonwealth v. Solis, 407 Mass. at 401-402, where there is a credible claim13 that the jury may have been exposed to an extraneous matter or influence, but there is also a claim of a Fidler violation that suggests wrongdoing by the defendant, the judge must conduct a meaningful inquiry into the alleged Fidler violation and weigh both claims in deciding whether the defend*773ant’s motion for juror inquiry should be granted. A meaningful inquiry may well require an evidentiary hearing in order to evaluate whether a Fidler violation occurred and, if so, its nature and extent.14 Where such an inquiry leads the judge to conclude that the Fidler principles in fact were violated by the defendant or someone acting on the defendant’s behalf, the next step is to balance the Fidler violation or violations against the information that has been presented about the extraneous matter or influence — accepting, for purposes of the balancing exercise, that such information is accurate. Based on the evidence received, if the judge continues to believe that there is a colorable showing of extraneous influence on the jury, and also finds a violation of Fidler by the defendant or on his behalf, the judge must balance these considerations in deciding whether to grant the defendant’s motion for juror inquiry. The balance ultimately struck will be highly dependent on the factual circumstances.15

The motion judge did not follow such a course in this case.16 Rather, after deciding that the defendant had raised a colorable claim of extraneous influence, the judge followed the two-step *774procedure prescribed by Fidler itself, requiring the defendant to show by a preponderance of the evidence that the jury were exposed to an extraneous matter or influence, and then shifting the burden “to the Commonwealth to show beyond a reasonable doubt that [the defendant] was not prejudiced by the extraneous matter.” Commonwealth v. Kincaid, 444 Mass. 381, 386 (2005), quoting Fidler, 377 Mass. at 201. At the hearing held on the Commonwealth’s motion for reconsideration, the motion judge did allow the parties to present evidence on the issue of extraneous influence, including the testimony of the trial judge, court officers, and the defendant, but he did not permit the Commonwealth to cross-examine the defendant about his postverdict contacts with Constantine or any other steps the defendant may have taken to contact jurors. The motion judge also suggested to the Commonwealth that he would not permit Constantine to testify.17

In the circumstances here, this was error. The judge appeared to have credited Juror A’s testimony that Constantine had made three telephone calls to Juror A over a period of several months. These contacts have a ring of overzealous persistence, if not harassment, and warranted further exploration — both as to their nature and as to the defendant’s involvement in, or acquiescence to, Constantine’s actions. It was important to investigate this issue, because Constantine’s actions appeared to strike at the heart of a principal aim of the Fidler proscriptions: preventing harassment of jurors and potential interference with the integrity of the jury system.

Because the motion judge did not permit the Commonwealth a meaningful opportunity to inquire of Constantine or the defendant about the alleged Fidler violations, we remand the case to him for further proceedings.18 After conducting further inquiry, the judge should conduct the balancing analysis described above *775in order to decide whether the defendant is entitled to a new trial.19

b. Claim relating to proof of extraneous influence. We address briefly a separate challenge that the Commonwealth has raised in its appeal from the judge’s decision because it may have bearing on the proceedings on remand.20 The Commonwealth argues that the judge’s finding of extraneous influence was clearly erroneous as a factual matter. Findings are clearly erroneous only if the reviewing court has a “firm conviction that a mistake has been committed.” Commonwealth v. Kincaid, 444 Mass. at 384, and cases cited. Kendall v. Selvaggio, 413 Mass. 619, 620-621 (1992), quoting J.A. Sullivan Corp. v. Commonwealth, 397 Mass. 789, 792 (1986). To the extent the Commonwealth asserts that the motion judge was somehow obliged to believe the trial judge and court officers over the jurors, “[t]he determination of the weight and credibility of the testimony is the function and responsibility of the judge who saw and heard the witnesses, and not of this court.” Commonwealth v. Moon, 380 Mass. 751, 756 (1980). The motion judge gave the trial judge’s statements that he did not enter the jury room minimal weight, noting that “to a judge cases often blur over time especially if there is a delay from the time of a trial to the point where one is called upon to recollect the event.” There is no basis in the record on which to reject this credibility determination. Notably, Juror A was the only juror contacted by Constantine, yet other jurors testified that the trial judge had *776entered the jury room. Three of the five testifying jurors (Jurors A, C, and D) testified that the trial judge was in the jury room during deliberations without counsel or the defendant present; a fourth juror (Juror E) remembered somehow receiving an answer to a question during the deliberations that was not in writing or given in the court room; and a fifth juror (Juror B), who stated that the trial judge did not enter the jury room, did not have a strong memory of the trial.21

The motion judge did not make specific findings as to what the trial judge said in the jury room, but he concluded, with ample support in the record, that the jurors were given the impression that they were required to reach a verdict and that deadlock was not an option. A trial judge’s entering a jury room before the jury have returned a verdict and providing instructions off the record to jurors, without the presence of the defendant or counsel, may constitute an improper extraneous influence on a jury.22 See United States v. United States Gypsum Co., 438 U.S. 422, 459-462 (1978) (granting new trial in part because judge had ex parte conversation with jury foreman during deliberations and foreman likely received impression that judge wanted verdict “one way or the other”); Sargent v. Roberts, 1 Pick. 337, 341 (1823) (granting new trial because “no communication whatever ought to take place between the judge and the jury, after the cause has been committed to them by the charge of the judge, unless in open court, and, where practicable, in presence of the counsel in the cause”); Commonwealth v. Patry, 48 Mass. App. Ct. 470, 475 (2000) (granting new trial because judge gave supplemental instructions to jury in jury *777room, without presence of defendant, and stating, “[W]e think that judges should not enter jury rooms at any time to conduct the court’s business, even with the parties’ consent or at the invitation of the jury. Jury rooms are often small, cramped, and designed to hold a limited number of people. Because of the closeness, the proceedings can easily descend into a sense of informality and intimacy that may not be conducive to the proper administration of justice”).

There is nothing in the record to indicate that the trial judge intended to influence the course of the jury’s deliberations. Nevertheless, his mere presence in the jury room may have influenced the jury’s decision-making. Although the instruction about continuing to deliberate until reaching a verdict may have been correct if given in the court room as part of the trial judge’s charge prior to the start of the jury’s deliberations, the potential for confusion was heightened because the instruction may have been given in response to a juror question about what to do if the jury could not reach a verdict, and because it was given within the confines of the jury room — subtly leading the jurors to infer that they must reach a verdict in order to leave the room.23

3. Conclusion. For the reasons stated, we vacate the order *778granting the defendant’s motion for a new trial and remand the matter to the District Court for further proceedings consistent with this opinion before the motion judge.

So ordered.