Prosecutors called their first witnesses Tuesday, Oct. 13, in the federal trial of Matthew Farwell, a former Stoughton police officer accused of killing Sandra Birchmore at her Canton apartment in 2021. The trial is expected to last about five weeks. The government alleges Farwell killed Birchmore after she told him she was pregnant and said he was the father, and then staged her death as a suicide. DNA evidence later showed Farwell was not the unborn child’s biological father. The allegations have not been proven in court. Prosecutors also allege Farwell began sexually exploiting Birchmore when she was 15 and involved in Stoughton’s police Explorers program, where he served as an instructor. They say the sexual relationship continued after she became an adult. Stoughton Police Officer Heather Cheever, the first witness, said Birchmore looked up to officers and sought her advice about life and a possible law enforcement career. Cheever described herself as a mentor. She said she had not heard that Farwell was tutoring Birchmore at a library and would have questioned it if she had. Under cross-examination, Cheever said she could not recall exactly how often she saw Birchmore during overnight shifts. She testified that the FBI first asked her about Birchmore’s death more than three years after it occurred. Cheryl Carlson, a former assistant principal at East Elementary School in Sharon, said she hired Birchmore as an instructional assistant in 2020. Carlson testified that Birchmore’s last day at work was Feb. 1, 2021. After Birchmore did not respond to school emails when classes resumed on Feb. 4, staff contacted a school resource officer to request a welfare check. Carlson said she learned that day that Birchmore had died and that the death was being described as a suicide. She testified that this did not fit with her impression of Birchmore, who had seemed excited about having a baby. Erin Porter, who managed Birchmore’s apartment complex, said Birchmore began renting there on Dec. 1, 2020, and told her she was pregnant. Porter described Birchmore as kind and said she seemed excited. Porter was working on Feb. 4, when police asked for help entering Birchmore’s apartment. Porter testified that she stayed outside and did not go in. Canton police officer Ensley Cotard said he went to the apartment for a welfare check after a school resource officer called. He and the property manager entered after knocking and receiving no answer. Cotard testified that he did not see a suicide note and did not collect evidence; a medical examiner was called. Carlos Verissimo, the apartment complex’s maintenance supervisor, testified that he helped police obtain surveillance footage. He said the building had one camera, in the lobby, and that it recorded Birchmore entering while texting and Farwell entering later. Verissimo also said residents sometimes propped open the building’s front doors despite signs asking them not to. Massachusetts State Police Trooper Mathew Dunne also testified, and was expected to return Wednesday. The state investigation did not result in criminal charges in the case; federal prosecutors later brought charges after reaching a different conclusion about Birchmore’s death. The judge told jurors she would instruct them before graphic photographs and other evidence were shown, emphasizing that their decision must not be driven by emotion. The federal trial is not being filmed in the courtroom.