Federal prosecutors are asking to call three witnesses who allegedly had relationships with former Stoughton police officer Matthew Farwell, arguing their testimony could help jurors assess why he wore a mask when he went to Sandra Birchmore’s apartment on the night she died. In a Sunday filing, prosecutors renewed a request to present the testimony at Farwell’s federal trial. A judge had earlier declined to admit it, but left open the possibility that it could become relevant depending on the defense’s arguments, prosecutors said. Farwell, 40, is charged with killing Birchmore, 23, and causing the death of her unborn child in February 2021. Prosecutors allege he killed Birchmore after she told him he was the baby’s father; DNA evidence later showed he was not. Farwell’s attorney, Kimberly Stevens, has argued Birchmore died by suicide. Both sides addressed Farwell’s mask during opening statements. Prosecutors said he wore it to avoid being identified near the apartment. Stevens argued he was trying to keep his wife from learning about an affair with Birchmore. Prosecutors say testimony from the three proposed witnesses would show Farwell did not take similar steps to conceal those alleged relationships. Their accounts would be limited to when the relationships began, how long they lasted and whether Farwell tried to keep them secret, the filing said. Prosecutors also said the witnesses would testify that he did not wear face coverings during their relationships or ask them to delete messages. In a separate filing, prosecutors asked the judge to give jurors corrective instructions about the legal requirements for convicting Farwell of killing a witness to a federal crime and about the need for a unanimous verdict. The government disputed the defense’s description of what prosecutors must prove, saying the case does not require proof that Birchmore was planning to contact a federal officer directly. Prosecutors said they must establish a reasonable likelihood that a relevant communication, if made, would have gone to a federal law enforcement officer. Prosecutors also objected to the defense’s mention of evidence they say is disputed or not included in any noticed exhibit. They specifically challenged a claim that Birchmore had texted a friend, “I was gushing blood,” saying no such message exists. The government asked the judge to caution the defense against raising disputed or unintroduced evidence during witness testimony. The trial was scheduled to resume Tuesday, Oct. 13, with the 10 men and six women on the jury expected to hear from prosecution witnesses.