Matthew Farwell's twin testifies at trial, says he had sexual contact with Sandra Birchmore as on-duty officer

Ex-Stoughton, Massachusetts, police officer William Farwell testified at his twin brother Matthew Farwell's murder trial on Monday that he had sexual contact with former cadet Sandra Birchmore while on duty.

William Farwell said Birchmore was between the ages of 20 and 21 during their sexual relationship. Matthew Farwell is on trial for allegedly killing Birchmore and her unborn child and covering it up to make her death look like a suicide. Prosecutors say Matthew Farwell began grooming Birchmore when she was a 13-year-old in the Stoughton Police Department's Explorers program and started having sex with her when she was 15.

William Farwell began testifying in federal court in Boston late Monday afternoon and will be back on the stand Tuesday. He acknowledged to the jury that he's testifying under a grant of immunity by the government.

William Farwell testifies in Sandra Birchmore case

"My understanding is that my statements may not be used against me," Farwell said. "I could be charged with a criminal offense for having sexual relations while on duty as a police officer."

Farwell, who became a Stoughton police officer in 2017, said he does not remember having any interactions with Birchmore before she turned 18. He said she reached out to him on Facebook Messenger asking for his help reenlisting in the military after being discharged for medical reasons.

The prosecutor asked Farwell if he knew back in July of 2018 that Birchmore and his brother Matthew had a sexual relationship. William Farwell said he did not know that at the time.

In the following weeks, Farwell testified that "we began a consensual sexual relationship," saying they would meet in either one of their cars or in the house where Birchmore lived with her aunt.

William Farwell had his police certification permanently revoked in 2024. A prior internal investigation by the Stoughton Police Department found that he had an inappropriate physical relationship with Birchmore and exchanged explicit messages and photos with her while on duty.

Medical examiner testifies about Sandra Birchmore's death

Before William Farwell took the stand Monday, the jury heard from a medical examiner whose initially ruled that Birchmore died by suicide.

Dr. Maria Capo Martinez testified that when she first received the case, her office was told there were no signs of trauma and that Birchmore was found hanging by a scarf in her Canton apartment with no suicide note.

But Birchmore was actually found with a duffel bag strap around her neck, and prosecutors pointed out that her autopsy photos revealed a number of possible abrasions and even a possible blunt force injury on her chest. 

The medical examiner performed Birchmore's autopsy the day after her body was found in 2021. She said that when she went to finalize her ruling, she checked back in with Massachusetts State Police Lt. John Fanning.

"Nothing new to report. We have some forensics out at the lab. We can discuss the case at any time if needed," Fanning replied.

That means Capo Martinez was never shown the final surveillance video of Birchmore or Matthew Farwell, told about laundry in Birchmore's machines or her excitement about becoming a mother.

It wasn't until two years later when the FBI got involved and the case was reevaluated that the medical examiner revisited her conclusions. Capo Martinez eventually changed Birchmore's death certificate, switching the cause of death from asphyxia by hanging to asphyxia, and the manner of death from suicide to undetermined.

The prosecution could rest their case in court this week. Matthew Farwell is being charged with a federal count of killing a witness or victim and causing the death of an unborn child. He has pleaded not guilty.