The FBI arrested a former Massachusetts police officer on allegations he groomed a teenage girl, continued to have sex with her when she was an adult and killed her in her apartment when she told him she was pregnant with his child, staging it to look like a suicide. 

A federal indictment says Matthew Farwell, 38, killed Sandra Birchmore, 23, to prevent authorities from finding out details of his sexual offenses. Farwell was arrested at a shopping plaza in the city of Revere Wednesday after he was indicted on charges he strangled Sandra Birchmore in early 2021 after she told him she was pregnant and that he was the father. 

Farwell worked as an officer for the police department in Stoughton, south of Boston, for 10 years, from 2012 until 2022. It was not immediately clear why he left the department.

Birchmore began participating in the police explorers program when she was 12 years old, according to the indictment in U.S. District Court in Massachusetts. Court documents say Farwell, who was a police explorers volunteer, used his authority and access to groom, sexually exploit and then sexually abuse Birchmore when she was 15 and that he continued to have sex with her when she became an adult.

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“During some of the shifts when Farwell was supposed to be performing his duties as a Stoughton police officer, he was instead engaged in sex acts with Birchmore,” the indictment says. 

“Farwell committed this murder, at least in part, to prevent Birchmore from disclosing to authorities information regarding his commission or possible commission of one or more crimes, including coercion and enticement, deprivation of rights under color of law and a then-ongoing wire fraud scheme,” FBI Special Agent Chenee Castruita wrote in an affidavit. 

“Farwell’s murder of Birchmore was the culmination of Farwell’s years-long pattern of abuse targeting Birchmore.” 

In late 2020, Birchmore found out she was pregnant and told Farwell, according to the indictment.

At a news conference Wednesday, FBI Assistant Special Agent in Charge Stephen Kelleher said Birchmore was excited when she found out she was pregnant and texted Farwell a poster she made celebrating the pregnancy. She had also reached out to a friend about baby clothes and to a photographer about potential baby photos, according to Kelleher. 

“Matthew Farwell’s gun and badge did not grant him authority to violate the Constitution, and it certainly didn’t entitle him to sexually exploit, abuse and rape a child before killing her and her unborn baby in an attempt to cover up his alleged crimes,” Kelleher said, calling the allegations against Farwell “depraved.”

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Investigators also believe Farwell staged the apartment to make it look like she had died by suicide, Kelleher said.

The next month, a friend of Birchmore’s called the Stoughton Police Department to tell officials Farwell had been having sex with Birchmore. Farwell allegedly strangled Birchmore on about Feb. 1, 2021, and then used his police knowledge to stage her apartment to make it look as though she had killed herself, according to the indictment.

On some occasions, Farwell had been on duty when he sexually abused Birchmore as a minor, and he falsely claimed certain work hours to hide that offending behavior, court documents say.

“Farwell continued this sexual abuse after Birchmore became an adult, and he continued to engage in an extramarital sexual relationship with Birchmore until he killed her on February 1, 2021,” Castruita wrote. 

Stoughton Police Chief Donna McNamara said Wednesday the department had worked with other agencies, including the FBI, to investigate.

“The day after Sandra Birchmore was found dead in her Canton apartment, I ordered a lengthy and aggressive internal affairs investigation, the instructions of which made it clear that no stone should be left unturned,” McNamara said in a statement. “The alleged murder of Sandra is a horrific injustice. … 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.”

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Farwell pleaded not guilty during an initial appearance in federal court in Boston Wednesday. He was handcuffed and led out of the room. A detention hearing is scheduled for Sept. 10, and another hearing is scheduled for Oct. 17.

If convicted of killing a witness or victim, Farwell would face a minimum sentence of life in prison. Acting U.S. Attorney Joshua Levy on Wednesday declined to comment on whether federal authorities would seek to impose the death penalty if Farwell is found guilty, saying the decision would be made by the Department of Justice, according to The Associated Press. 

Massachusetts has outlawed capital punishment. 

The Associated Press contributed to this report.