The first words of Matthew Farwell’s defense were: “Matthew Farwell is a man who had an affair.” Two days later, prosecutors used that sentence to ask the judge for something she had already refused them once. In a motion filed Sunday, October 11, the government asked Chief Judge Denise Casper to let three women testify about their own alleged extramarital affairs with Farwell, and about one detail in particular: he never wore a mask.
The motion, which we read in full on the court docket, is a case study in how a defense opening can open the door to evidence that would otherwise stay out. WCVB and MassLive also reported the filing. Here is what prosecutors want, the rule they are invoking, and why it matters for the night of February 1, 2021. Farwell has pleaded not guilty and is presumed innocent. The judge has not ruled.
What the government is asking for
According to the motion, Docket No. 338:
- A second try. The government had earlier moved, under seal, to admit “limited testimony proving that the defendant engaged in at least three extramarital affairs during the relevant period.” The judge denied it, but at oral argument “cautioned that it may revisit the issue if the defendant’s evidence or statements to the jury gave the evidence … some special relevance.”
- What changed. Prosecutors point to three lines from the rough transcript of Friday’s defense opening by attorney Kimberly Stevens: “Matthew Farwell is a man who had an affair”; that Farwell wore a mask on February 1, “which is what people having affairs might do”; and that his “wife is not a federal law enforcement officer.”
- What the women would say. The government says the three women “will say Farwell never took steps to hide their encounters. He never wore a mask or disguise. In fact, he had a cavalier attitude about their encounters.” It proposes to ask each about the existence and length of the relationship, their relationship to Farwell’s wife, whether he tried to hide the affair, whether he ever told them to delete messages, and whether he ever wore a mask or disguise.
- The point. “The evidence will prove Farwell never cared about his wife discovering his infidelity, so he wore a mask on Feb. 1 for another reason: not to be seen at the apartment where Ms. Birchmore lay dead,” the motion says. The deleted-messages question, prosecutors argue, shows Farwell treated Birchmore differently because she was “the victim of his sexual abuse at 15 years of age.”
The women are not named in the public filing.
The rule: Federal Rule of Evidence 404(b)
Evidence of other conduct is ordinarily kept out of a criminal trial. Rule 404(b)(1) says evidence of “any other crime, wrong, or act is not admissible to prove a person’s character in order to show that on a particular occasion the person acted in accordance with the character.” Prosecutors cannot argue that because a defendant cheated on his wife, he is the kind of person who would kill.
But Rule 404(b)(2) allows the same evidence “for another purpose, such as proving motive, opportunity, intent, preparation, plan, knowledge, identity, absence of mistake,” among others. In the First Circuit, the government notes, courts apply a two-step test: whether the evidence has “special relevance” to an issue other than character, and whether its probative value is “substantially outweighed by the danger of unfair prejudice” under Rule 403. The motion cites United States v. Hicks, 575 F.3d 130 (1st Cir. 2009).
How a defense opening can open the door
The government’s central argument is that the defense created the relevance. It relies on United States v. Landry, 631 F.3d 597 (1st Cir. 2011), in which, as the motion describes it, the First Circuit upheld admission of previously excluded evidence after the defendant opened on a theory of good faith, because the defense “made the evidence specially relevant.”
The logic in this case runs like this:
- The defense told jurors the mask and other “bizarre conduct” were what a cheating husband does to avoid getting caught.
- If Farwell never hid his other affairs, the government argues, the cheating-husband explanation for the mask fails.
- That leaves the government’s explanation: he hid his face because of what happened in the apartment.
On unfair prejudice, the government argues there is little left to protect: “Because his chosen defense is that he had an affair, there is no unfair prejudice in the jury learning more about his three other affairs.” It also notes that “adult infidelity carries far less stigma than it did in the past,” and that a limiting instruction would tell jurors not to reason that “because Farwell is a cheater, he is also a murderer.”
What the defense is likely to argue
As of Monday, the docket showed no defense response. Defendants in this position typically argue that the evidence is exactly the character inference Rule 404(b)(1) forbids, that three additional affairs would put Farwell’s personal life on trial, and that Rule 403’s danger of unfair prejudice substantially outweighs any value. The judge already excluded this evidence once, so the question is narrow: whether Friday’s opening changed the balance.
Why this fight matters
The government must prove that Farwell killed Birchmore and did it to prevent a communication about federal crimes to a federal officer. 18 U.S.C. § 1512(a)(1)(C). Prosecutors have said building video shows Farwell entering her building in a mask and hood on February 1, 2021. If jurors accept the defense explanation, the mask is evidence of an affair. If they hear from three women that he never hid anything from them, the mask looks like something else. That makes this ruling one of the most consequential evidentiary decisions in the trial.
Frequently asked questions
Why do prosecutors want women who had affairs with Matthew Farwell to testify?
To rebut the defense explanation that Farwell wore a mask to Sandra Birchmore’s apartment because he was hiding an affair from his wife. The government says the three women will testify he never hid his relationships with them or wore a mask or disguise.
Has the judge allowed the testimony?
Not yet. Judge Casper denied an earlier version of the request but said she might revisit it depending on the defense case. The government renewed the motion on October 11, 2026.
What is Rule 404(b)?
A federal rule of evidence that bars using other acts to prove a person’s character and that he acted in character, but allows such evidence for other purposes, such as motive, intent, plan or knowledge, subject to Rule 403’s balancing of probative value against unfair prejudice.
When does testimony begin in the Farwell trial?
Tuesday, October 13, 2026, in federal court in Boston.
Sources: Government’s Motion in Limine to Admit “Specially Relevant” Evidence Regarding the Defendant’s Extramarital Affairs, United States v. Farwell, No. 1:24-cr-10259-DJC (D. Mass.), ECF No. 338, filed October 11, 2026, read in full via CourtListener’s RECAP archive; quotations from the defense opening are from the rough transcript as quoted in that motion. WCVB and MassLive, October 12, 2026. Federal Rule of Evidence 404 read at the Legal Information Institute. Landry and Hicks are described as the government characterizes them. The government’s statements are arguments; the defense has not yet responded publicly and the court has not ruled. Matthew Farwell is presumed innocent. General information about the law, not legal advice.