On June 25, 1979, three men robbed a branch of the Berkshire Bank and Trust Company in Pittsfield. Following a jury trial, the defendant was found guilty of armed robbery while masked and sentenced to life imprisonment for his participation in the robbery. The defendant timely filed a notice of appeal. Approximately a year after his conviction, the defendant filed a motion for new trial, based on the prosecutor’s failure to obtain and produce on request exculpatory evidence held by the Federal Bureau of Investigation (F.B.I.). Following a hearing, a judge of the Superior Court denied the motion for new trial. The defendant filed a notice of appeal from that denial. The appeals from the original trial and from the denial of the motion for new trial were consolidated in the Appeals Court. We transferred the case here on our own motion.
The defendant raises two issues on appeal. First, the defendant argues that the motion judge erred in concluding that the prosecutor was not obliged to obtain material and exculpatory evidence in the possession of the F.B.I. and to provide the evidence to him prior to trial. Second, the defendant argues that he was prejudiced by the prosecutor’s closing argument to the jury.1 We conclude that the defendant was denied a fair trial by the unavailability of material and exculpatory evidence and that a new trial must be ordered. Because we order a new trial on this ground, we do not consider the propriety of the prosecutor’s closing argument.2
The Commonwealth’s case. In order to place in perspective the facts surrounding the undisclosed evidence, we first review the Commonwealth’s case as it may have appeared to the jury. On June 25, 1979, around 11:00 a.m., three men disguised with wigs, beards, and hats robbed the Allendale branch of the Berkshire Bank and Tmst Company, taking $259,710.90. *592At .least one of the men was armed. The three men ran out of the bank and entered a tan station wagon. Shortly thereafter, they switched to a red Oldsmobile automobile. Both vehicles were found. From the tan getaway car, the police recovered a laundry bag with about $11,000 in it, some of which was “bait” money taken from the bank. In the Oldsmobile, the police found eyeglasses, a hat, a wig, other clothes, and some personal papers.
At trial, the Commonwealth’s theory was that the defendant robbed the bank with two other men, Ralph Petrozziello and Kenneth Wightman. Several witnesses testified about the presence in Berkshire County in late June, before the robbery, of a group of persons that included, at different times, the defendant, Petrozziello, Wightman, and two others. Only two of the five witnesses so testifying were able to identify the defendant; one witness testified that she had seen the defendant washing a red Oldsmobile. Two bank employees identified the defendant as a person they might have seen at the bank at some time before the robbery, but were not able to identify the defendant as one of the robbers. Four bank employees identified Petroz-ziello as one of the robbers. Another witness testified to seeing the robbers leave the bank and identified the defendant as the driver of the getaway car. The witness stated that he and the defendant had looked at each other for a period of thirty to forty seconds from a distance of twenty to forty feet. The witness testified that all he could see at the time of the robbery was the suspect’s nose and part of his cheek. He testified that a month after the robbery he had picked the defendant’s photograph out of an array solely on the basis of his “odd . . . real flat” nose.
The remaining evidence linking the defendant to the robbery was provided by one Ronald Gates, who was with the defendant in jail while the defendant awaited trial. Gates testified that the defendant admitted to him, in the presence of two other inmates, that he robbed the bank. The defense called these other two inmates to the stand, and both denied Gates’s story. Another inmate testified that the defendant and Gates stayed away from each other.
*593The missing alibi evidence. Prior to trial the defendant informed the district attorney that he intended to offer an alibi defense that he was in a cabin in the Berkshires while the bank robbery in question was being committed. The defendant said that Rita Haggerty and Deborah Sperrazza would support his alibi. Defense counsel was unable to locate Haggerty, but did learn that Sperrazza was in the Federal Witness Protection Program. Upon the allowance of the defendant’s motion for the physical production of a witness, the assistant district attorney telephoned the United States Attorney in Springfield and obtained his assistance in producing Sperrazza. In September, 1980, shortly before trial, United States marshals brought Sper-razza to the district attorney’s office in Pittsfield. She refused to speak with defense counsel, but did talk with the assistant district attorney. She told him that she and the defendant had come to Berkshire County to rob a bank. They checked escape routes, and the defendant stole a police radio scanner from a local store. Sperrazza also stated that on the morning of the robbery, she was in a cottage in Lanesboro with the defendant, Petrozziello, and Wightman. She said that she was asleep up to the time when Red Halliday came to the cottage with the information that the bank had been robbed. In response to questioning, she said that she could not say whether the defendant was present at the cottage at all times before she awoke. The assistant district attorney promptly informed defense counsel of these statements. Defense counsel concluded that Sper-razza’s testimony would not be helpful at trial and that she should not be called as a witness.
Before trial, the defendant also subpoenaed F.B.I. Agent Joseph Adams, requiring him to produce all records of his investigation of the bank robbery. On the morning of trial, Adams met defense counsel at the courthouse. He refused to divulge the contents of his investigative file, but did inform defense counsel that he had nothing which would “help you” in connection with the defense.
About a year after the defendant’s conviction, he learned that, before his trial, Sperrazza had given a statement to the *594F.B.I. that exculpated him.3 The defendant thereupon moved for a new trial on the basis of the suppression of the report of that interview. At the hearing on the motion, the F.B.I. produced a report of a second interview with Sperrazza.
The motion judge found that in November, 1979, the F.B.I. twice interviewed Sperrazza and made transcribed report's (“302 reports”) of the interviews. Relevant portions of the reports dated November 5, 1979,4 and November 21, 1979,5 *595are set out in the margin. The statements were similar to the ones Sperrazza was to give ten months later, but differed in several important respects. Sperrazza stated that on the morning of the robbery, she was the first out of bed before the defendant, Wightman, and Petrozziello.6 She also reported that upon learning of the robbery, the three men were very “upset” and that immediately after the robbery, the defendant had no money.
The motion judge found that the 302 reports containing Sperrazza’s statements were material and exculpatory: “In essence, the alibi to be offered the jury would be that while the defendant was contemplating and preparing a robbery of the bank, someone else beat him to it.” The judge also found that the prosecutor “made a full and complete disclosure of all exculpatory evidence that was in the possession and custody of its office and the existence of which was known to the Commonwealth at all relevant times up to and including the trial of the defendant,” but that the district attorney’s office *596had no knowledge of the F.B.I. interviews with Sperrazza until the defendant filed his postconviction motion for a new trial.
Discussion. “Ordinarily the prosecutor’s obligation to disclose information is limited to that in the possession of the prosecutor or police.” Commonwealth v. Liebman, 379 Mass. 671, 675 (1980) (Liebman I), citing Commonwealth v. Campbell, 378 Mass. 680, 702 (1979). This rule follows from Brady v. Maryland, 373 U.S. 83 (1963), where the Supreme Court held that “the suppression by the prosecution of evidence favorable to an accused upon request violates due process where the evidence is material either to guilt or to punishment, irrespective of the good faith or bad faith of the prosecution.” Id. at 87. The prosecutor cannot be said to suppress that which is not in his possession or subject to his control. Because the prosecutor apparently had no control over the F.B.I. 302 reports, it has not been shown that he suppressed them and, thus, the first requirement for proving a Brady violation has not been established here. Commonwealth v. Liebman, 388 Mass. 483, 487 (1983) (Liebman II).
Nevertheless, the motion judge found that the evidence was material and exculpatory.7 Moreover, it is clear that, if the 302 *597reports had been in the possession of State authorities, the defendant would have been entitled to them under our law; if the prosecution had been in the Federal system, the defendant would have been entitled to the reports under Federal law. See, e.g., United States v. Gaston, 608 F.2d 607, 612-614 (5th Cir. 1979); United States v. Bryant, 439 F.2d 642, 650 (D.C. Cir. 1971). As we noted in Liebman I, the “introduction of two sovereignties creates a potentiality for unfairness which would need correction if realized in practice.” Id. at 674. In Liebman I, we held that such unfairness should be avoided by “assimilation of State and Federal proceedings.” Id. Thus, the precise issue presented here is whether, in the circumstances of this case, the principles of Commonwealth v. Liebman I and II oblige the prosecutor to try to obtain material and exculpatory evidence in the possession of the F.B.I. The principles and policies underlying Brady are useful guides for our analysis. See Liebman II, supra at 487.
Brady mandates some cooperation by the prosecutor with defense counsel in the interests of a fair trial, but the departure from a pure adversary model is limited. United States v. Bagley, 473 U.S. 667, 675 n.6 (1985). The prosecutor is not expected to investigate all exculpatory evidence that may exist, Commonwealth v. Stone, 366 Mass. 506, 511 (1974); to produce exculpatory evidence held by government agencies other than the prosecutor and police, Commonwealth v. Campbell, supra; or to give discovery of every aspect of his case, Commonwealth v. Adrey, 376 Mass. 747, 754 (1978); Weatherford v. Bursey, 429 U.S. 545, 559-561 (1977). Thus, the Brady rule is tempered by continued adherence to the adversary system as a means of discovering the truth and by the practical concern that the prosecutor not be so burdened that the administration of justice is unduly hampered. For these reasons, a showing that Federal authorities held material and exculpatory evidence without more is insufficient to justify a departure from the general rule limiting the prosecutor’s obligation to disclose to evidence in the possession of the Commonwealth at the time of trial. Commonwealth v. Gilday, 382 Mass. 166, 174 (1980).
*598We have recognized, however, that, in some circumstances, the prosecutor should be required to seek access to material and exculpatory evidence. See Liebman I, supra. In Liebman I, the defendant sought access to the minutes of Federal grand jury testimony that allegedly contained exculpatory evidence. He had been unsuccessful in petitioning the Federal courts for an order granting access to the minutes. Had the grand jury been a State grand jury, or had the prosecution been brought in a Federal court, the defendant would have been entitled to the material he sought. Id. at 674. We held that, when the defendant successfully moves in State court for production of Federal grand jury minutes, the cooperative relationship between State and Federal prosecutors and the existence of Federal precedent for ordering the release of grand jury minutes in connection with State proceedings justified placing on the State prosecutor the burden of securing Federal cooperation. Id. at 675. Thus, in Liebman I, a strong potentiality of unfairness to the defendant, the defendant’s inability otherwise to obtain the evidence in question, and the relatively light burden on the prosecutor weighed in favor of placing the responsibility on the prosecutor. When the potentiality for unfairness in fact was realized, we determined that the defendant should be afforded a new trial. Liebman II, supra at 486-490.
Similar considerations have arisen in cases involving the failure of State prosecutors to obtain on request the F.B.I. “rap sheets” (records of prior convictions) of victims and witnesses. See Briggs v. Raines, 652 F.2d 862, 865 (9th Cir. 1981); Martinez v. Wainwright, 621 F.2d 184, 187 (5th Cir. 1980); State v. Ireland, 11 Or. App. 264, 268 (1972). The courts in those cases appear to have been influenced by the fact that F.B.I. rap sheets are unavailable to defense counsel directly but are available to local police as a matter of course. “FBI rap sheets are in the constructive possession of the state in almost every case.” Briggs v. Raines, supra at 865, quoting State v. Ireland, supra at 268. See Martinez v. Wainwright, supra at 185 n.2 & 187.
The degree of cooperation between Federal and State investigators on a particular case has also been an important factor *599in determining the prosecutor’s obligations. In United States v. Antone, 603 F.2d 566 (5th Cir. 1979), that court found that, where State and Federal investigators had cooperated extensively, the State investigators functioned as agents of the Federal government and that their knowledge of the perjury of the government’s principal witness should be imputed to the Federal prosecutors. Id. at 570. Similarly, in Commonwealth v. Manning, 373 Mass. 438 (1977), we held that the misconduct of a Federal agent working in close cooperation with State investigators had interfered with the defendant’s right to counsel and therefore required dismissal of the State indictment.
In summary, the Liebman cases and similar cases suggest that the following factors are important in determining whether the prosecutor is obligated to seek requested exculpatory evidence from Federal authorities: the potential unfairness to the defendant; the defendant’s lack of access to the evidence; the burden on the prosecutor of obtaining the evidence; and the degree of cooperation between State and Federal authorities, both in general and in the particular case.
Following the principles of Liebman I and II, we believe that in this case the prosecutor was obliged at least to request the F.B.I. to forward any material, exculpatory evidence in its possession. The same unfairness that existed in Liebman II is present here. The 302 reports would have been available to the defendant if the prosecutor or local police had physical possession of them, or if the defendant had been prosecuted in the Federal system.8 The defendant unsuccessfully attempted to obtain the reports on his own.9 The defendant subpoenaed the *600files of the F.B.I. agent who worked on the bank robbery. The agent, though refusing to honor the subpoena, told defense counsel that he did not have any exculpatory information in his files anyway. In addition, the defendant was unable to locate Sperrazza to interview her during most of the pretrial period because she was in the Federal Witness Protection Program. Once Sperrazza was located, some ten months after the interviews, she refused to speak with defense counsel.
By contrast, the 302 reports may well have been available to the prosecutor on request. The first of the two reports in fact came to light in the course of discovery of another Commonwealth criminal prosecution,10 and the F.B.I. voluntarily produced the second report at the hearing on the motion for new trial.
We have noted that the motion judge found the cooperation between Federal and State investigators in this case to have been limited. Thus, there is no ground for imputing the F.B.I.’s knowledge or for attributing the apparent misrepresentations of the F.B.I. agent to the Commonwealth. However, “cooperation between State and Federal prosecutors is and should be common enough,” Liebman I, supra at 675, so that a State prosecutor would not be unduly burdened by passing on to the appropriate Federal authorities a defense request for exculpatory materials held by the F.B.I. At least in the circumstances presented here, where Federal authorities also investigated the case and a key witness is in the Federal Witness Protection Program, the prosecutor should have forwarded the request.
That burden is not relieved in this case by the absence of a court order directing the prosecutor to produce the F.B.I. reports. Soon after his arrest, the defendant specifically requested the prosecutor to obtain F.B.I. reports. The prosecutor made a complete disclosure of all exculpatory information in his possession; however, he neither asked the F.B.I. or the United *601States Attorney for such information, nor apparently did he indicate to defense counsel that he would not do so in the absence of a court order. The prosecutor’s cooperation made it unnecessary for the defendant to insist on a court order. In these circumstances, the defendant should not be penalized for failure to obtain a court order.11 However, in view of the absence of a court order and the limited cooperation between State and Federal law enforcement authorities in this case, the prosecutor’s only responsibility was to pass the request along to the appropriate Federal officials and to transmit the Federal response to counsel. Or, if he had a good faith reason for refusing to do so, the prosecutor should have so informed the defendant, so that he could seek a court order.12 We need not speculate on the further responsibilities, if any, the prosecutor may have had if the Federal authorities had declined to cooperate.13
Finally, we reiterate that the motion judge found the 302 reports to be both exculpatory and material. From the record, it appears that the Commonwealth’s theory of the case was that the defendant robbed the bank with Wightman and Petroz-ziello. Much of the evidence pointed to Wightman’s and Pet-rozziello’s whereabouts before the robbery and to identification of Petrozziello as one of the robbers.14 Had the 302 reports *602been timely provided to the defendant, he may have chosen to call Sperrazza as a witness to construct the alibi defense that he did in fact come to Berkshire County with Wightman and Petrozziello to rob a bank, but that somebody else beat them to it. Such an alibi would have been consistent with the Commonwealth’s evidence linking the three men together before the robbery and with the identification of the defendant by two tellers as someone they might have seen in the bank before the robbery. Most importantly, that alibi would have been material because, if believed, it would have been a complete defense. Although we find it difficult to disagree with the motion judge that such an alibi might have been dangerous to use at trial, the decision whether or not to offer that alibi was a strategic one for defense counsel and the defendant to make. It is inappropriate for a judge on a motion for new trial to assume the role of counsel and determine whether and to what use particular evidence should have been introduced, or to assume the role of the jury and pass on the credibility of a witness and her story. Liebman II at 489.
Conclusion. On the defendant’s specific request for exculpatory materials in the possession of the F.B.I., the prosecutor was obliged either to seek the cooperation of the appropriate Federal authorities or, if he had a good faith reason for refusing to do so, to inform the defendant of that refusal. Because the prosecutor’s failure to take either of these steps denied the defendant access to material and exculpatory evidence, the defendant is entitled to a new trial.
The judgment is reversed, the verdict set aside, and the case is remanded to the Superior Court for a new trial. The order denying the motion for a new trial is reversed.
So ordered.