Prosecutors Seek to Call Affair Partners in Matthew Farwell Murder Trial

Federal prosecutors filed a new motion on Sunday asking a judge to allow testimony from three women who allegedly had extramarital affairs with former Stoughton police officer Matthew Farwell during his ongoing federal murder trial in Boston. Farwell, 40, is accused of killing 23-year-old Sandra Birchmore and causing the death of her unborn child in February 2021, staging the death as a suicide at her Canton apartment. Farwell’s defense contends that she died by suicide.

Federal Prosecutors Seek to Introduce Extramarital Affair Testimony

Federal prosecutors are attempting to reverse an earlier pre-trial ruling that barred testimony regarding Farwell’s alleged affairs. In a filing submitted on Sunday, October 11, 2026, the government argued that defense attorney Kimberly Stevens opened the door to this evidence during her opening statements on Friday by claiming Farwell wore a mask on the night of Birchmore’s death solely to hide an extramarital affair from his wife. “The defendant chose not to heed the Court’s warning,” federal prosecutors wrote. Prosecutors contend that Farwell’s behavior with other partners directly contradicts this defense.

According to court filings, the proposed witnesses will testify that Farwell never wore face coverings or disguises during their relationships, never asked them to delete messages, and maintained a cavalier attitude about their encounters. In fact, the government wrote, he had a cavalier attitude about their encounters. Prosecutors stated that Farwell took no steps to conceal those relationships from his wife, arguing that his efforts to remain hidden on the night of Birchmore’s death were intended to cover up a crime rather than an ordinary infidelity. Farwell’s adult affair partners will offer critical testimony that strikes at the heart of Farwell’s disclosed defense and Attorney Stevens’s purported innocent explanations for Farwell’s mask-wearing, they said.

The trial is scheduled to resume on Tuesday morning before a jury consisting of 10 men and six women. Testimony from prosecution witnesses is expected to begin as the court weighs whether to admit the disputed testimony about the three affair partners.

Disputed Legal Interpretations and Disputed Evidence in Opening Statements

Alongside the motion regarding the affair partners, prosecutors filed a separate request asking the judge for curative jury instructions. The government argued that defense counsel made significant misstatements of law regarding the elements required for a guilty verdict.

Farwell is charged in federal court with killing a witness of a federal crime. Prosecutors stated that Stevens misconstrued the statute by suggesting the government must prove Birchmore was seeking to communicate directly with a federal officer. The government countered that it must only show a reasonable likelihood that a relevant communication would have been made to a federal law enforcement agency, noting the underlying crimes Farwell allegedly sought to conceal included deprivation of rights under color of law, coercion and enticement, and wire fraud.

Prosecutors also objected to references made by the defense regarding evidence not present in the trial exhibits. Specifically, the government noted that defense counsel told the jury Birchmore texted a friend stating she was “gushing blood,” a message prosecutors assert does not exist in any noticed exhibit. The government asked the court to warn the defense against referencing disputed or unnoticed evidence during ongoing witness testimony.

Legal Context and Community Impact of the Stoughton Case

The prosecution’s case centers on allegations that Farwell killed Birchmore to prevent her from revealing ongoing sexual exploitation that allegedly began when she was a minor. Michael Coyne, chief legal analyst for NBC10 Boston, noted that video and still photographs of Farwell wearing a mask near the apartment are highly damaging evidence that the jury will review repeatedly. Coyne said that prosecutors want to sort of dispel the narrative that he had any other reason to wear that make other than to cover up his criminal activity. The video, the still photos of that, are very, very damaging, Coyne said.

The trial resumes Tuesday with the presentation of further evidence and witness testimony.

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