BOSTON — A federal jury in Boston on Tuesday convicted former Stoughton police officer Matthew Farwell of killing Sandra Birchmore and causing the death of her unborn child, the U.S. Attorney's Office in Massachusetts said.

Jurors found Farwell guilty on both counts, the office said in a post on X. He was charged with killing a witness or victim and with causing the death of a child in utero, Patch reported.

Farwell faces an automatic life sentence, The Boston Globe reported. Federal prosecutors did not seek the death penalty.

The verdict came a day after the jury of eight men and four women sent a note telling Chief U.S. District Judge Denise Casper it was deadlocked. When jurors returned Tuesday morning, Casper encouraged them to take as much time as they needed to reach a verdict, the MetroWest Daily News reported. Deliberations began Friday after more than three weeks of testimony from 50 witnesses, according to the Globe.

A death first ruled a suicide

Prosecutors said Farwell strangled Birchmore, 23, in her Canton apartment on Feb. 1, 2021, and staged the scene to look like a suicide by hanging, WBUR reported. The state medical examiner, relying partly on information from police, initially ruled her death a suicide, according to NBC Boston. Federal prosecutors took up the case three years later, CNN reported.

Prosecutors argued that Farwell killed Birchmore to keep her from telling authorities that he had sexually abused her when she was 15, according to WBUR. Farwell was a police officer and instructor in the Stoughton Police Explorer program, a youth program Birchmore took part in, GBH News reported. Under federal law, jurors had to find that Farwell killed her specifically to stop her from reporting a federal crime to authorities, NBC Boston reported.

Among the key evidence was surveillance video from the lobby of Birchmore's apartment building showing her, and then Farwell wearing a mask and a hood, heading to her apartment the day she died, according to NBC Boston. Prosecutors also pointed to DNA evidence they said linked Farwell to the strap found around Birchmore's neck, Boston 25 News reported. Prosecutors said the killing came the day before Farwell's third child was born.

Farwell pleaded not guilty. His lawyers acknowledged he had an extramarital relationship with Birchmore but argued she took her own life after he ended it that night and told her he was not the father of her baby, according to GBH News.

Birchmore and Farwell had both believed he was the father. DNA testing later showed the father was Jason Pickard, an Army Reserve sergeant who testified at the trial, the Signal reported in October.