An ex-detective is accused of killing and staging the death of a woman he abused — years after it was ruled a suicide

A Massachusetts cop is accused of grooming a teenage girl he mentored, beginning a years-long sexual relationship with the child, then strangling her to death in 2021 after she told him she was pregnant.

Former Stoughton Police Department Det. Matthew Farwell successfully “staged” the murder scene to look like 23-year-old Sandra Birchmore had died by suicide , according to a bombshell indictment unsealed Wednesday in Massachusetts federal court.

FBI agents arrested Farwell, 38, on Wednesday morning, US Attorney Joshua Levy said at an afternoon press conference in Boston . The stunning twist follows a finding in June by a pathologist hired by Birchmore’s family, who contended her death had in fact been a homicide .

Farwell killed Birchmore to silence her about his criminal actions, the indictment says, calling Birchmore’s violent death “the culmination of Farwell’s years-long pattern of abuse.”

“The alleged murder of Sandra is a horrific injustice,” Stoughton Police Chief Donna McNamara said in a statement on Wedneaday. “The allegations against the suspect, a former Stoughton Police Officer, represent the single worst act of not just professional misconduct but indeed human indecency that I have observed in a nearly three-decade career in law enforcement.”

The four-page indictment handed down against Farwell is accompanied by a 45-page detention memo filed by prosecutors and contains one sordid claim after the next. In one text message, Birchmore tells Farwell that she “loved” spending the night with him instead of attending her high school prom, and that it “was worth not going.” In another, when Birchmore would have been 15 and Farwell would have been around 27, she “recalled the specific class and seat that she was in when she realized that she was going to lose her virginity to Farwell.” In that same conversation, Farwell said it was “a big thing to get to be [her] first,” according to the detention memo.

Birchmore joined the Stoughton PD Police Explorers, a vocational program for middle- and high school students to learn more about law enforcement careers, in 2010 at the age of 12, the indictment states. In 2012, Farwell, who had then been a cop for four years with the Wellesley, Massachusetts PD, joined the force in Stougton, it says. Farwell not only served as a police officer, but did double duty as an instructor for the Explorers program, the indictment explains.

Things began with a friend request on Facebook from Farwell to Birchmore, after which the alleged abuse began. The sexual abuse transpired in various settings, including a Five Guys parking lot while Farwell was on duty, according to the memo. To cover his tracks, Farwell allegedly fudged his time sheets to make it appear he had been on the job when in fact he was “engaging in sex acts” with a minor, court papers say. An internal investigation found that three Stoughton PD officers had had inappropriate sexual relations with Birchmore , among them, Farwell’s brother William.

In late 2020, Birchmore, then 23, learned she was pregnant and told Farwell she was excited to become a mother, according to the indictment. She said she expected Farwell to be involved, to which he later responded by putting her in a headlock and telling her he wished she were dead, the indictment alleges.

On January 20, 2021, less than two weeks before Birchmore would turn up dead, a friend of hers called the Stoughton PD and revealed that she had been engaged in an illicit sexual relationship with Farwell, according to the indictment.

Farwell found out about the call, and by the end of January 2021, he “faced the specter of additional disclosures to law enforcement regarding his prior sexual abuse of Birchmore and his then-ongoing efforts to defraud the Stoughton Police Department and Town of Stoughton by meeting her for sex when he was supposed to be on duty as a police officer,” the detention memo alleges.

“On February 1, 2021, Farwell went to Birchmore’s apartment in Canton, Massachusetts wearing a COVID-19/medical facemask,” it states. “Once inside, he killed her and staged her body and apartment to make it appear that Birchmore had died by suicide. On February 4, 2021, three days after the murder, members of the Canton Police Department went to Birchmore’s apartment to conduct a well-being check. When police officers entered Birchmore’s apartment, they found her body.”

Farwell resigned in 2022 amid an ongoing probe. Earlier this year, he voluntarily gave up his peace officer certification and agreed to never again work in law enforcement.

Levy on Wednesday declined to specify if a single piece of evidence led investigators to focus on Farwell as a suspect in Birchmore’s death, saying, “We get leads on cases all the time from all different sources. I don’t want to comment on how we get information, or how we got into this one.” In a press release on Wednesday, the Department of Justice said the feds “probed old evidence and revealed critical new evidence leading to the charge against Farwell.”

A police officer being arrested in such a case is significant, and Levy said he doesn’t know exactly how uncommon it is. But, he said, “it’s an unusual situation, and that’s partly why it’s so incredibly disturbing.”

Farwell now faces one federal count of killing a witness or victim. If convicted, he faces a mandatory sentence of life in prison or the death penalty.