Former Massachusetts detective indicted on federal charges he killed a young woman who claimed he fathered her unborn child

(CNN) — A former Massachusetts police detective has been indicted in connection with the killing of a young pregnant woman federal prosecutors say he began sexually exploiting when she was a teen in a law enforcement youth program.

Matthew Farwell, 38, who was a detective in the town of Stoughton, was indicted on a count of killing a witness or victim relating to the 2021 strangulation death of 23-year-old Sandra Birchmore, according to the US Attorney’s Office for the District of Massachusetts. He is also accused of staging her body and apartment to make it appear as a suicide, the office said.

Farwell was arrested Wednesday and pleaded not guilty during an initial court appearance. His next appearance is September 10. CNN has reached out to his attorney for comment.

Farwell is accused of killing Birchmore to prevent anyone from reporting a federal crime to law enforcement, including coercing or enticing a minor to engage in sexual activity, according to the indictment.

“When it became clear that Mr. Farwell … could no longer control Sandra Birchmore, he allegedly silenced her permanently,” acting US Attorney for Massachusetts Joshua Levy told reporters Wednesday. “He allegedly attempted to cover his tracks to literally try and get away with murder. And he almost did.”

An officer with the Stoughton Police Department since 2012, Farwell was an instructor with the department’s Police Explorers Academy law enforcement program for young people, according to the indictment. Birchmore had joined the program in 2010 at the age of 12, and Farwell “allegedly used his authority and access to groom, sexually exploit and ultimately sexually abuse” her by the time she was 15, a court affidavit said.

Some of his sexual contact with the minor occurred while he was on duty, according to the indictment, and Farwell is accused of continuing to have sex with Birchmore until her death in February 2021.

Federal prosecutors said the young woman learned she was pregnant in December 2020. She told family members she was happy and started making doctors’ appointments and purchasing items for her unborn child, according to the court documents. In January 2021, a friend of Birchmore’s called the police department and told an employee about the officer’s sexual contact with Birchmore. A department employee later alerted Farwell, who was married, to the call, the documents said.

Prosecutors say when Birchmore told Farwell he was the father of the unborn child and expected him to be involved, he became violent, including one assault in which he placed her in a headlock and “told her that he wished she were dead.”

On February 1, 2021, Farwell went to Birchmore’s apartment in Canton, with his face covered in a Covid-19 medical mask, according to court documents.

“Once inside, he killed her and staged her body and apartment to make it appear that Birchmore had died by suicide,” the affidavit said. Her body was discovered during a wellness check on her apartment three days later. A medical examiner ruled her death a suicide.

After Birchmore’s death, law enforcement agents went over her cell phones and a laptop and discovered messages between her and Farwell, according to court documents. The messages revealed Farwell “had sex with Birchmore when she was a minor,” the documents said. There were also exchanges about having unprotected sex.

In court documents, prosecutors accuse Farwell of killing Birchmore – the pregnant woman “he had exploited and used for years, by strangling her to death. Within minutes of silencing Birchmore, video surveillance footage shows him leaving her apartment.”

The count of killing a witness or victim has a mandatory sentence of death or life in prison, according to prosecutors.

Stoughton Police Chief Donna McNamara said in a statement Wednesday that after Birchmore was killed, she “ordered a lengthy and aggressive internal affairs investigation, the instructions of which made it clear that no stone should be left unturned.”

“The alleged murder of Sandra is a horrific injustice,” McNamara said. “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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CNN’s Celina Tebor contributed to this story.