Federal prosecutors allege ex-Mass cop Matthew Farwell had three extra-marital affairs

Colleen Cronin, Boston Herald on Oct 12, 2026

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BOSTON -- Federal prosecutors are renewing a motion to call three witnesses who allegedly had affairs with ex-Stoughton cop Matthew Farwell to the stand during his federal murder trial.

A judge had ruled against admitting that testimony before the trial began, the government wrote in its Sunday filing, but left open the door for the evidence to be admitted if it became relevant because of the defence’s argument.

“The defendant chose not to heed the Court’s warning,” federal prosecutors wrote.

Farwell, 40, is accused of killing 23-year-old Sandra Birchmore and causing the death of her unborn child in February 2021. Prosecutors alleged that Farwell killed Birchmore after she told him he was the father of her baby, something that DNA evidence later disproved. Farwell’s defense contends that she died by suicide.

During opening statements Friday, both sides mentioned that Farwell arrived at Birchmore’s apartment on the night of her death wearing a mask. Prosecutors said it was to prevent anyone from identifying Farwell near the crime scene, while Farwell’s attorney Kimberly Stevens argued that her client was trying to hide his affair from his wife.

“Attorney Stevens opened this trial by telling the jury that Farwell’s bizarre conduct was merely evidence of an innocent man’s desire to hide his infidelity from his wife,” prosecutors wrote in their motion. “Farwell chose that defense. The consequence is the government must disprove it with evidence.”

By calling the other alleged “affair partners,” prosecutors argued they’ll be able to show that Farwell didn’t go to the same lengths to conceal their relationships and didn’t care whether his wife found out.

“Farwell only cared about — and took steps to prevent — people from discovering his criminal conduct with Sandra Birchmore,” they wrote.

The proposed witnesses will testify that Farwell, “an ardent anti-masker,” never wore face coverings during their relationships and never requested that they delete their messages with him, prosecutors wrote.

“In fact,” the government wrote, “he had a cavalier attitude about their encounters.”

The filing noted that the witnesses would be questioned on a limited scope, including about when the relationships started, how long they lasted, and what efforts, if at all, Farwell took to conceal them.

“Now that Attorney Stevens has chosen to tell the jury that Farwell cheated on his wife and wore a mask to prevent her from finding out about it, the government should not be barred from introducing the evidence that directly rebuts those statements,” the government wrote.

In a separate filing, prosecutors also requested curative instructions to the jury to address, “two significant misstatements of law regarding the elements of the offense and verdict unanimity,” made by Stevens during her opening.

Farwell has been charged in federal court with killing a witness of a federal crime, and the government argued that Stevens misconstrued the elements prosecutors need to prove for a guilty verdict.

Prosecutors wrote Stevens’ opening made it seem that the government had to prove Birchmore was seeking to communicate directly with a federal officer to find her client guilty of killing a witness.

But the government argued, for a guilty verdict, “it must show that there was a reasonable likelihood that a relevant communication, had it actually occurred, would have been made to a federal law enforcement officer.”

According to prosecutors, the crimes Farwell sought to conceal were deprivation of rights under color of the law, coercion and enticement, and wire fraud.

The motion also argued that Stevens mentioned exhibits during her opening that are still disputed and evidence that isn’t a part of any of the exhibits, disputed or not.

“Defense counsel stated in opening that Ms. Birchmore told a friend, ‘I was gushing blood,’” the government wrote. “Such a text message does not exist. It certainly is not in any exhibit the defendant has noticed.”

Prosecutors asked that the judge warn against asking about any of the disputed evidence during witnesses’ testimony.

“There is no way to un-ring the bell that defense counsel rang during their opening statement, but the government requests that the Court strongly caution defense counsel from attempting to reference disputed or unnoticed evidence again,” they wrote.

Farwell’s murder trial is set to resume Tuesday morning, with the 10 men and six women on the jury expected to hear testimony from prosecution witnesses.