BOSTON — When Sandra Birchmore was found dead in her Massachusetts home, police quickly decided it was a suicide. From the start, her friends didn’t believe it.

The 23-year-old had recently learned she was pregnant — and was thrilled about it, they told police. The person upset about the pregnancy, they said, was the married police detective she’d been seeing, Matthew Farwell.

Five years later, Farwell, 40, is on trial in federal court, charged with killing Birchmore and staging her death to look like a suicide to hide that he had begun sexually abusing her when she was 15.

Also on trial, in a sense, is the original police investigation.

State police troopers working in the Norfolk County district attorney’s office never wavered from their judgment that Birchmore’s death was a suicide, according to federal prosecutors. Farwell was interviewed for just 15 minutes in a parking lot, they said. Investigators didn’t take notes, examine him for injuries or sample his DNA. Police did look at Farwell’s phone, but only after he’d deleted his texts with Birchmore.

“Not only did they not investigate, they very quickly told others, including the medical examiner, that this was definitively a suicide and there was no foul play,” Assistant U.S. Attorney Torey Cummings told jurors.

Among those they reassured was Farwell’s twin brother, Billy. He testified that the lead investigator — a longtime family friend — told him he wanted the case closed. Billy Farwell recalled the investigator saying that he needed the investigation like he needed a “bullet in the head.”

One of Farwell’s attorneys, Kimberly Stevens, acknowledged to jurors Farwell “had an affair” but insisted he is no killer.

“He has made regrettable mistakes. He is human,” Stevens told jurors. “Matthew did not kill Sandra Birchmore.”

The FBI began looking into Birchmore’s death in 2023, partly at the urging of a former deputy chief at the Stoughton Police Department, where Farwell worked.

Deputy Chief Brian Holmes had led an internal affairs investigation, but Farwell resigned before Holmes released findings that concluded Farwell had a sexual relationship with Birchmore.

An expert brought in by federal authorities, Dr. William Smock, concluded Birchmore was strangled with a duffel strap and that she struggled before dying.

Birchmore was 12 when she participated in a police mentoring program in Stoughton for children interested in careers in law enforcement. Farwell, who was 24, soon friended her on Facebook.

Years later, they would exchange 40,000 texts, some of which were read at the trial. In one 2020 text, Birchmore told Farwell she would never forget the day they first had sex: April 10, 2013, when she was 15 and Farwell was 27.

“Best day of my life,” Birchmore wrote.

“Me too,” Farwell replied.