Three men were tested for gunshot residue in the hours after a shooting at a Dorchester parade. All three tests came back negative. The Commonwealth’s own expert told the jury that a positive result requires at least three particles, and that there is “nothing to be drawn” from a negative result.
The Commonwealth then had that same expert tell the jury that one particle was found on Keith Williams’s hand and one on his T-shirt, and none on the other two men, and argued in closing that this showed Williams was the shooter.
On Monday the Supreme Judicial Court threw out every conviction in the case.
What the court held
Commonwealth v. Williams, SJC-12702, was released September 22, 2026, argued last December. Justice Dewar wrote for a panel of Chief Justice Budd and Justices Gaziano, Wendlandt, Dewar and Wolohojian.
The holding is narrow on its face and consequential in practice. The court agreed “that the Commonwealth failed to establish the reliability of using the subthreshold number of gunshot residue particles detected in a negative test result as evidence that the defendant was the shooter. To the contrary, as stated, the Commonwealth’s own expert testified that nothing could be drawn from the negative test results.”
Because the error was prejudicial, Williams gets a new trial on murder in the first degree, armed assault with intent to murder, and assault and battery by means of a dangerous weapon.
Why the threshold exists
This is the part worth understanding, because it is what turns a technical evidentiary ruling into a rule about how forensic science is used.
The three-particle minimum is not bureaucratic caution. It exists, in the expert’s own testimony, “to account for the possibility of contamination and to ensure accurate interpretation.” Gunshot residue is transferable. It sits on police car seats, on handcuffs, on the hands of officers who have been to a range. One or two particles is the number at which the laboratory cannot tell a shooter from someone who stood near a shooter, or near a police cruiser.
So when the Commonwealth offered one particle and one particle as proof of who fired, it was offering exactly the reading the threshold was designed to prevent. The SJC’s point is not that gunshot residue evidence is junk. It is that a methodology is only reliable for the question it was validated to answer.
The standard the Commonwealth had to meet
Expert testimony in Massachusetts is governed by Daubert v. Merrell Dow Pharmaceuticals, 509 U.S. 579 (1993), as adopted in Commonwealth v. Lanigan, 419 Mass. 15, 26 (1994). The touchstone is reliability, and it is reliability as applied to the specific question the expert is answering, not reliability of the technique in the abstract. The opinion runs through Commonwealth v. Hinds, 487 Mass. 212 (2021), and Commonwealth v. Patterson, 445 Mass. 626 (2005), for the proposition that appellate review of an admissibility ruling is deferential but “not perfunctory,” and that a judge’s findings “must apply the correct legal standard to the facts of the case.”
The trial judge had let the evidence in on the reasoning that the defense objections went to weight rather than admissibility, that cross-examination could address them, and that the jury would decide what inferences to draw. That is a common and usually sound instinct. The SJC’s answer is that it does not work where the proponent has not established reliability for the use being made of the evidence in the first place. Weight is what a jury assesses after the gatekeeping is done.
Because the defense preserved its objection at trial, Williams needed only to show prejudice, meaning the court could not say the error “did not influence the jury, or had but a slight effect.”
The second holding, which will outlast the first
The court separately vacated the conviction for carrying a firearm without a license, “without need for further discussion,” because the jury was not instructed that the Commonwealth bore the burden of proving the defendant did not have a valid license. It cited Commonwealth v. Carleton, 497 Mass. 11, 13 (2026), which in turn cites Commonwealth v. Guardado, 493 Mass. 1, 12 (2023).
Guardado shifted the burden on licensure from the defendant to the Commonwealth, and it has been quietly unwinding firearm convictions ever since. Three years on, it is still producing vacaturs in cases tried before it was decided. Any defense lawyer with a pre-2023 firearm conviction on a direct appeal or a new trial motion should be reading that paragraph.
What this does not mean
Williams was not exonerated, and the opinion says so in terms.
The court rejected his sufficiency argument and held the evidence of deliberate premeditation adequate, so he may be retried for murder in the first degree. The rest of the Commonwealth’s case was substantial: although no witness identified him as the shooter, he was seen nearby disposing of the murder weapon seconds after the shooting, in the company of two men whom witnesses placed at the scene. One witness described the shooter as the largest of three men running from the scene, and Williams was distinctly taller and heavier than his two companions.
Dawn Jaffier was killed by a stray round as she crossed Blue Hill Avenue on the morning of August 23, 2014. Lealah Fulton was struck in the leg while sitting on the median watching the parade. Neither was a target. A retrial means that case is tried again, twelve years on.
How many other cases does this reach
Fewer than the headline suggests, and more than zero.
The ruling does not disturb positive gunshot residue results at or above the laboratory threshold. It reaches the narrower and, until now, quietly common practice of putting a subthreshold particle count in front of a jury and inviting the inference that the person with one particle is the person who fired.
The Commonwealth had leaned on Commonwealth v. Johnson, 463 Mass. 95, 108 (2012), in seeking admission. Practitioners who have been relying on that citation for the same move should read Williams closely, because the reliability showing it now requires was not made here and will be hard to make anywhere.
The people with a live claim are defendants whose trials featured that specific argument and whose cases are still on direct appeal or within reach of a new trial motion. That is a real but limited set, and identifying it requires reading trial transcripts rather than laboratory reports, because the error is in how the number was used, not in the test.
What did the SJC decide in Commonwealth v. Williams?
That the Commonwealth failed to establish the reliability of using a subthreshold gunshot residue particle count from a negative test as evidence of who fired a weapon. The error was prejudicial, and all of the defendant’s convictions were set aside.
Why does a positive gunshot residue test require three particles?
According to the Commonwealth’s own expert, to account for the possibility of contamination and to ensure accurate interpretation. Below that threshold the laboratory cannot reliably distinguish a shooter from someone exposed to residue another way.
Can Keith Williams be tried again?
Yes. The court rejected his argument that the evidence of deliberate premeditation was insufficient, so retrial on murder in the first degree is permitted.
Why was the firearm conviction vacated?
Separately, because the jury was not instructed that the Commonwealth had to prove he lacked a valid firearm license, following Commonwealth v. Guardado and Commonwealth v. Carleton.
Does this mean gunshot residue evidence is inadmissible?
No. The ruling addresses using particle counts below the laboratory’s own positive threshold to identify a shooter. It does not address results at or above that threshold.
Who decided the case?
Justice Dewar wrote the opinion. The panel was Chief Justice Budd and Justices Gaziano, Wendlandt, Dewar and Wolohojian. It was argued December 8, 2025 and released September 22, 2026.
All quotations, holdings, facts, dates and citations taken from the slip opinion in Commonwealth v. Williams, SJC-12702, released September 22, 2026, which we read in full. Daubert v. Merrell Dow Pharmaceuticals, 509 U.S. 579 (1993), Commonwealth v. Lanigan, 419 Mass. 15 (1994), and Commonwealth v. Latimore, 378 Mass. 671 (1979) verified on CourtListener. The reporter citations for Commonwealth v. Hinds, 487 Mass. 212 (2021), Commonwealth v. Guardado, 493 Mass. 1 (2023), Commonwealth v. Carleton, 497 Mass. 11 (2026), Commonwealth v. Patterson, 445 Mass. 626 (2005), Commonwealth v. Johnson, 463 Mass. 95 (2012) and Commonwealth v. Colas, 486 Mass. 831 (2021) are given as the SJC gives them in this opinion; CourtListener has no record for several of them yet. Keith Williams’s convictions have been set aside and he is presumed innocent on retrial. General information about Massachusetts law, not legal advice.