A Stoughton police officer is expected to be the first witness Tuesday in the federal trial of Matthew Farwell, a former Stoughton police officer accused of killing Sandra Birchmore, whose death in Canton was initially ruled a suicide. Birchmore, 23, was found dead in her apartment in February 2021. Federal prosecutors allege Farwell strangled her on Feb. 1 and staged the scene to look like a suicide. They say Birchmore was pregnant and believed Farwell was the father. Prosecutors say Farwell met Birchmore when she was 13 and taking part in a Stoughton police youth program. They allege he groomed her over several years and began having sex with her when she was 15 and he was 27. Prosecutor Torey Cummings said in opening statements Friday that Birchmore had threatened to reveal information about Farwell, adding, “She could end his life with the information she had, so he was going to end hers first.” Farwell’s attorney, Kimberly Stevens, told jurors, “Matthew Farwell did not kill Sandra Birchmore. He did not.” The defense has acknowledged an affair but argued that Farwell did not believe the baby was his and that Birchmore died by suicide after he ended the relationship. A defense court filing says DNA testing showed Farwell was not the father. Farwell faces a federal charge of killing a witness or victim, which carries a mandatory life sentence if he is convicted, as well as a charge related to the death of an unborn child. He has pleaded not guilty to both charges. Prosecutors have also asked the court to allow testimony about three other alleged affairs involving Farwell, a married father of three. They argue the evidence would show he took additional steps to conceal his relationship with Birchmore. Sixteen jurors—10 men and six women—have been seated. The trial is expected to last about five weeks. Cameras are not allowed in federal court, and proceedings are not being livestreamed or broadcast.