STAHL, Circuit Judge.

In this appeal, defendant-appellant Trent Manning challenges, on several grounds, his convictions for possession with intent to distribute cocaine, use of a firearm during and in relation to a drug trafficking crime, and possession of a firearm by a convicted felon. Manning's principal appellate claim is that improper comments made by the prosecutor during closing arguments undermined the fairness of his trial. After carefully reviewing the record, we conclude that the prosecutor's comments did so infect the proceedings below that Manning is entitled to a new trial. Accordingly, confining our discussion and analysis to the prosecutorial misconduct issue, we vacate the convictions.

I. BACKGROUND

A. Relevant Factual Background

It is undisputed that late in the afternoon on October 7, 1991, several members of the Providence Police Department executed a search warrant at Manning's mother's house, located at 151 Doyle Avenue in Providence, Rhode Island. In the course of their search, the police officers seized, inter alia, a brown briefcase which contained the material that formed the basis of the charges in the indictment: two bags of cocaine weighing 124.64 grams, various drug paraphernalia, a loaded .9 millimeter handgun, and six copper pipe bombs.

It also is undisputed that just prior to the raid, two men drove up to Manning's mother's house in Manning's red Jeep Cherokee. The man in the passenger seat was one Sean Duncan, who was detained by the police but never charged in connection with this case. The vehicle's driver, however, was not apprehended at the scene. The identity of the driver was (and is) perhaps the most hotly contested issue in this case, as it was (and is) the government's theory that the driver brought the brown briefcase into the house. The government argued successfully that the driver was Manning; Manning and Duncan testified that the driver was one Troy McKenzie. The primary government witness on the issue of the driver's identity was Detective Joseph Lennon, a member of the search team that day. Lennon testified that, after having been given the order to execute the search warrant, he approached the rear of 151 Doyle Avenue, where he saw Manning, whom he knew and with whom he had conversed on other occasions, standing outside the Cherokee and in front of the house's garage. Lennon testified that Manning was holding the brown briefcase. Lennon also testified to seeing Duncan seated in the passenger seat of the Cherokee.

Lennon further testified that, upon seeing Manning, he identified himself as a police officer and, with gun drawn, ordered Manning to stop. According to Lennon's testimony, Manning ignored this directive, walked slowly into the garage, and closed and locked the door behind him. Lennon testified that his pursuit of Manning into the building was delayed by the presence of Manning's rottweiler, which was running around loose in the driveway area behind the house. After eventually gaining entrance to the garage (about three to five minutes later), Lennon found and seized the brown briefcase. He did not, however, find Manning in the house.

Detective David Lussier also testified concerning the identity of the driver of the Cherokee just prior to the raid. Lussier, who also had known Manning for some time, testified that he observed Manning, along with a companion, drive by his surveillance position (located about 50 yards from the house in a parking lot which provided a direct view into the rear yard of 151 Doyle Avenue) just three or four minutes before the raid. Indeed, Lussier testified that he ordered that the warrant be executed at that time precisely because he feared that eye contact between himself and Manning had caused his surveillance to be compromised. After ordering the raid, Lussier testified that he drove to the front of the house, entered it through the front door, and proceeded to the basement, where he found a broken window through which Manning apparently had escaped.

In contrast to the detectives' testimony, both Sean Duncan and Manning testified that the driver of the car at the time of the raid was (as noted above) Troy McKenzie. The substance of their testimony was that Duncan and Manning had been riding around together throughout the day, that they had picked up McKenzie at some point in the afternoon, and that they thereafter drove to the residence of Manning's girlfriend. At this point, Manning gave McKenzie and Duncan $40 and asked them to take his rottweiler, which was being kept at his mother's house, to the veterinarian for a rabies shot. McKenzie and Duncan then drove the Cherokee over to Manning's mother's house to pick up the dog. Duncan testified that McKenzie entered the house to fetch the dog while he waited in the car. Shortly thereafter, the raid occurred. Duncan also testified that he had "no idea" was happened to Troy McKenzie after he entered the house.

One week after the execution of the search warrant, Manning voluntarily turned himself in to the police. He subsequently was charged with and convicted of the crimes noted above, all of which necessitated a finding that Manning was the person in possession of the brown briefcase who disappeared into 151 Doyle Avenue at the time of the raid.

B. The Prosecutor's Comments

Manning complains of four different comments made by the prosecutor during the course of his closing argument. First, Manning contends that the prosecutor improperly vouched for the credibility of certain prosecution witnesses during the following colloquy:

[PROSECUTOR]: If Lussier is going to come in and lie to you he could have done that very, very easily. There's a million little ways they could have given it to the Defendant. But they cannot. The prosecution witnesses cannot engage in that kind of conduct. They're bound by the truth.

[DEFENSE COUNSEL]: I object to that, your Honor.

THE COURT: Overruled.

[PROSECUTOR]: They're bound by their oath and limits of honesty. The last thing you might ask yourselves —

[DEFENSE COUNSEL]: I object to that, again I have a motion.

THE COURT: Overruled, motion denied.

(Hereinafter "First Passage"). The government concedes that this passage contains improper witness-vouching by the prosecution.

Next, Manning argues that the prosecutor engaged in additional improper witness-vouching and inappropriately implied that he had additional incriminating evidence when, in responding to a defense argument concerning the lack of probative fingerprint evidence on the items in the brown briefcase, he stated:

[PROSECUTOR]: [W]hen we get to this gun and these bombs and this dope we've got an eyewitness who knows the Defendant and saw it all in his hands. So it doesn't matter whether there's a print on it or not. But they looked anyways and what did that BCI officer tell you? He told you that there were some partial prints on those items but nothing that was good enough to use for identification purposes. Nothing that has sufficient points of comparison on it for him to be positive and we have to be fair, we have to be positive. Prosecution —

[DEFENSE COUNSEL]: I object to that, Judge.

THE COURT: Overruled.

[PROSECUTION]: Prosecution must always be fair. . . .

(Hereinafter "Second Passage"). The government acknowledges that this prosecutorial argument also contained improper witness-vouching, but denies that it implied the existence of additional incriminating evidence. Rather, the government contends that it was an effort to suggest "that prosecution witnesses had not created false evidence[.]"

Third, Manning asserts that the prosecutor impermissibly appealed to the jury's emotions when, near the conclusion of the prosecutor's initial closing argument, the following exchange took place:

[PROSECUTOR]: Twelve responsible people will deliberate on this case. Take responsibility for yourselves. Take responsibility for your community.

[DEFENSE COUNSEL]: I object to that.

THE COURT: The jury's responsibility is to follow the Court's instructions and find the facts.

(Hereinafter "Third Passage"). The government denies that this argument constituted an improper appeal to the jury's emotions.

Finally, Manning charges that the prosecutor again impermissibly appealed to the jury's emotions at the conclusion of his rebuttal argument:

[PROSECUTOR]: Convict the Defendant fairly because the facts and the law compel conviction. Convict the Defendant because justice compels conviction.

[DEFENSE COUNSEL]: I object to that, too . . . .

THE COURT: I direct the jury to ignore the last statement of the United States Attorney. Your responsibility, as I told you at the beginning, is to determine whether or not, in light of the law that is given to you by the Court, the government has met its burden of proving the Defendant guilty beyond a responsible doubt . . .

(Hereinafter "Fourth Passage"). Although it is not entirely clear, the government appears to concede that this argument was improper. See Government's Brief at 41 (acknowledging that this argument "is somewhat similar to exhortations that have been deemed impermissible").

II. DISCUSSION

Manning's prosecutorial misconduct argument, as developed in his brief and at oral argument, proceeds along the following lines. First, Manning correctly notes that, in order to convict him of the crimes with which he was charged in the indictment, the jury was obliged, as a threshold matter, to find that he was the person who disappeared into the garage at 151 Doyle Avenue while holding the brown suitcase. Next, Manning observes that this finding necessarily must have been anchored upon determinations (1) that Detectives Lennon and Lussier were credible witnesses on this issue, and (2) that he and Duncan were not. Finally, Manning contends that the improper witness-vouching committed by the prosecutor, which was allowed by the trial court over his objection, both alone and in conjunction with the prosecutor's implication that there existed additional inculpatory evidence and the prosecutor's other inflammatory rhetoric, so compromised the jury's ability to make these essential and liminal credibility determinations that his trial was rendered fundamentally unfair. We find Manning's reasoning to be persuasive.

We begin by laying the groundwork for our analysis. First, we think that the First and Second Passages contain improper witness-vouching by the prosecution. See, e.g., United States v. Innamorati, 996 F.2d 456, 482 (1st Cir.) (prosecutor may not vouch for government witnesses), cert. denied, ___ U.S. ___, 114 S.Ct. 409, 126 L.Ed.2d 356 and ___ U.S. ___, 114 S.Ct. 459, 126 L.Ed.2d 391 (1993); United States v. Martin, 815 F.2d 818, 821-23 (1st Cir.), cert. denied, 484 U.S. 825, 108 S.Ct. 89, 98 L.Ed.2d 51 (1987). Next, we believe that the Third and Fourth Passages include improper appeals to the jury to act in ways other than as a dispassionate arbiter of the facts. See, e.g., United States v. Young, 470 U.S. 1, 17, 105 S.Ct. 1038, 1047, 84 L.Ed.2d 1 (1985) (prosecutor erred in telling jury to "do its job"); United States v. Mandelbaum, 803 F.2d 42, 44 (1st Cir. 1986) (prosecutor erred in urging jury to "do its duty"). Finally we think it rather obvious that, when read in context, the prosecutor's comments in the Second Passage were an attempt to counter defense counsel's anticipated argument relating to the lack of fingerprint evidence on the items seized from the brown briefcase. Insofar as the comments were intended to relieve the jury of any misapprehension that there were no fingerprints on these items, these comments were not improper. Given that defense counsel had not suggested that the partial prints were not Manning's, Cf. Young, 470 U.S. at 11-14, 105 S.Ct. at 1044-46 (discussing the "invited response" doctrine), however, the insinuation that the partial prints were inculpatory, which we believe inhered at least in the prosecutor's statement that "we have to be fair, we have to be positive," was impermissible. See e.g., United States v. Udechukwu, 11 F.3d 1101, 1106 (1st Cir. 1993) (prosecutor may not imply that the government has inculpatory information that is not in evidence); United States v. Smith, 982 F.2d 681, 683 (1st Cir. 1993) (similar).

We also feel constrained to express our belief that the government's overreaching in this case was not entirely limited to the prosecutor's conduct at trial. While we acknowledge that the government was forthright in admitting that the prosecutor engaged in impermissible witness-vouching, we are surprised at several of the other positions staked out in the government's appellate brief. First, we are hard-pressed to comprehend, let alone agree with, the government's characterization of the Second Passage, "in full context," as an effort to suggest "that prosecution witnesses had not created false evidence" (a claim the defense never made). Instead, as noted above, we think it plain (1) that the government was responding to defense counsel's anticipated argument that the jury should consider the absence of probative fingerprint evidence on the items found in the brown briefcase to be exculpatory, and (2) that, in stating that the prosecution "ha[s] to be fair" and "has to be positive," the prosecutor went too far and hinted that the partial prints tended to inculpate Manning.

The government's references to extra-circuit caselaw and strained attempt to distinguish the inflammatory rhetoric here from the cases cited by Manning also strike us as inappropriate. By now, we think it should be beyond question that, in this circuit at least, arguments urging a jury to act in any capacity other than as the impartial arbiter of the facts in the case before it are improper. See Mandelbaum, 803 F.2d at 44; cf. Arrieta-Agressot v. United States, 3 F.3d 525, 529-30 (1st Cir. 1993) (inflammatory arguments distract jury from the only issue presented in a case: whether the evidence establishes guilt beyond a reasonable doubt).

We turn now to our analysis. In this circuit, we have identified several factors relevant to a determination of whether prosecutorial misconduct has "'so poisoned the well,'" see United States v. Hodge-Balwing, 952 F.2d 607, 610 (1st Cir. 1991) (quoting United States v. Capone, 683 F.2d 582, 586-87 (1st Cir. 1982)), that a new trial is required. Although we have used slightly varying terminology in describing these factors, the common denominators are (1) the severity of the misconduct; (2) the context in which it occurred; (3) whether the judge gave any curative instruction and the likely effect of such instructions; and (4) the strength of the evidence against the defendant. See, e.g., Udechukwu, 11 F.3d at 1106; Arrieta-Agressot, 3 F.3d at 528-30; Hodge-Balwing, 952 F.2d at 610; United States v. Quesada-Bonilla, 952 F.2d 597, 601-02 (1st Cir. 1991); Capone, 683 F.2d at 585-86. These factors guide our conclusion as to whether the misconduct likely affected the trial's outcome. E.g., Udechukwu, 11 F.3d at 1106; Arrieta-Agressot, 3 F.3d at 528.

In this case, we think that all of these factors militate in favor of reversal. The prosecutorial overreaching that took place here, while certainly not conscience-shocking, was pervasive. Moveover, it occurred during closing arguments — the last words spoken to the jury by the trial attorneys — and in no way was provoked by improper arguments of defense counsel. Cf., e.g., United States v. Machor, 879 F.2d 945, 956 (1st Cir. 1989), cert. denied, 493 U.S. 1081, 110 S.Ct. 1138, 107 L.Ed.2d 1043 (1990) and 493 U.S. 1094, 110 S.Ct. 1167, 107 L.Ed.2d 1070 (1990).

More importantly, the district court not only failed to give curative instructions to counter the improper First and Second Passages, but it also tacitly indicated that the arguments in these Passages were proper by overruling defense counsel's contemporaneous objections to them. As a result, we think it likely that the jury inferred that Detectives Lennon and Lussier, as both law enforcement officials and prosecution witnesses, could properly be considered as having a heightened duty to testify honestly. Of course, such an inference undermines the impartiality with which the jury is supposed to make credibility determinations. Moreover, as we have stated, we think that the jury could have inferred from the "we have to be fair, we have to be positive" comments that the partial fingerprints on the items in the brown briefcase were Manning's.

This leads to our final and most important point. While there may have been abundant evidence in this case that Manning was a drug dealer and that the drug paraphernalia seized at 151 Doyle Avenue was Manning's, the question of whether Manning committed the crimes with which he was charged in the indictment (i.e., possession of the contraband items seized from the brown briefcase) turned entirely on whether, with regard to the issue of who carried the brown briefcase into 151 Doyle Avenue, the jury believed Detectives Lennon and Lussier or whether it believed Manning and Duncan. In our view, each of the witnesses gave a plausible account on this threshold question; that is to say, neither version of who was carrying the briefcase was inherently unlikely to be true. Given this, and given the further fact that we are precluded from making independent credibility determinations on appeal, see United States v. Alvarez, 987 F.2d 77, 83 (1st Cir.), cert. denied, ___ U.S. ___, 114 S.Ct. 147, 126 L.Ed.2d 109 (1993), the question before us really is whether the prosecutorial misconduct in the First and Second Passages (which, as we have stated, significantly interfered with the jury's ability to make an essential and liminal credibility determination) was likely to have affected the trial's outcome, see, e.g., Udechukwu, 11 F.3d at 1106; Arrieta-Agressot, 3 F.3d at 528. We are compelled to conclude that this question must be answered in the affirmative. Accordingly, Manning is entitled to a new trial.

III. CONCLUSION

Nearly sixty years ago, the Supreme Court stated:

The United States Attorney is the representative not of an ordinary party to a controversy, but of a sovereignty whose obligation to govern impartially is as compelling as its obligation to govern at all; and whose interest, therefore, in a criminal prosecution is not that it shall win a case, but that justice shall be done. As such, he is in a peculiar and very definite sense the servant of the law, the twofold aim of which is that guilt shall not escape or innocence suffer. He may prosecute with earnestness and vigor — indeed, he should do so. But, while he may strike hard blows, he is not at liberty to strike foul ones. It is as much his duty to refrain from improper methods calculated to produce a wrongful conviction as it is to use every legitimate means to bring about a just one.

Berger v. United States, 295 U.S. 78, 88, 55 S.Ct. 629, 633, 79 L.Ed. 1314 (1935). For the third time in the last six months, we find ourselves in the regrettable position of vacating a conviction because a United States Attorney has failed to honor sufficiently these precepts. See Udechukwu, 11 F.3d at 1106; Arrieta-Agressot, 3 F.3d at 530; see also United States v. Moreno, 991 F.2d 943, 949-50 (1st Cir.) (Torruella, J., dissenting) (arguing, inter alia, that the prosecutorial misconduct in that case warranted reversal of defendant's conviction), cert. denied, ___ U.S. ___, 114 S.Ct. 457, 126 L.Ed.2d 389 (1993).

Vacated and remanded.