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 Farwell. 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 bag 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.

Over time, the communications showed Farwell engaging in sexting that involved strangling or choking.

“I’ll grab your throat and say you won’t say a word to anyone or you will be sorry,” Farwell wrote in one message.

In 2020, prosecutors said Birchmore pressed Farwell to get her pregnant — and threatened to tell his wife about their relationship if he didn’t. After she became pregnant late that year, she texted Farwell an image that read, “Congrats, we are going to be parents.”

Farwell responded that he had “nothing to say.”

When Birchmore suggested in a text that Farwell should be present when the baby was born, he replied: “You are truly the worst person on the face of the earth.”

One of Birchmore’s friends, Leslie Davis, testified that the two fought. She said Birchmore told her about one fight where Farwell pushed her to the ground. Birchmore had wanted Farwell to promise to identify himself as the baby’s father on a birth certificate.

“He wouldn’t sign it and said, ‘That’s not my baby,’” Davis said.

In January 2021, a woman called Stoughton police and reported Farwell’s relationship with Birchmore.

Farwell was incensed, saying in a text to Birchmore, “You have no idea how bad what she did is.”

Twelve days later, on Feb. 1, 2021, Farwell texted Birchmore, asking to stop by her apartment. She said he was welcome.

Security camera video showed Farwell entering the building at 9:14 p.m. A digital forensics expert testified that Birchmore’s phone stopped moving at 9:40 p.m., three minutes before the lobby security camera recorded Farwell leaving.

Prosecutors say that during that time, Farwell killed Birchmore to prevent her from revealing their relationship. She was about eight weeks pregnant.

“It was no longer a matter of whether his crimes would be exposed; it was a matter of when,” Cummings, the prosecutor, told jurors. “He had lost control of Sandra. He had a real problem. He needed a way to silence her permanently.”

Farwell’s lawyer, Stevens, told jurors that Birchmore was a troubled woman who started taking psychiatric medication at age 6, attempted suicide as a teenager and was desperate after becoming pregnant. Birchmore was alone for the holidays and had less than $3 in the bank, Stevens said.

“She is worried. She is stressed. She is freaking out,” Stevens said.

Farwell, she said, went to Birchmore’s apartment to break up with her, not kill her.

The state’s medical examiner, relying partly on information from police, initially determined Birchmore died by suicide. The office has since changed the official cause of death to “undetermined.”

A paternity test revealed Farwell was not the one who impregnated Birchmore. It was Army recruiter Jason Pickard, who said he met Birchmore in 2019, when she was considering enlisting, and soon after started a sexual relationship. He testified that Birchmore told him Farwell was the father and that their relationship began when she was underage.

Prosecutors finished presenting their case Wednesday.

The defense was expected to call witnesses for a third day Friday before the case goes to the jury as early as next week.