When Sandra Birchmore was found dead in her Massachusetts home, police quickly decided it was a suicide. From the start, her friends didn’t believe it.

The 23-year-old had recently learned she was pregnant — and was thrilled about it, they told police. The person upset about the pregnancy, they said, was the married police detective she’d been seeing, Matthew Farwell.

Five years later, Farwell, 40, is on trial in federal court, charged with killing Birchmore and staging her death to look like a suicide to hide that he had begun sexually abusing her when she was 15.

Also on trial, in a sense, is the original police investigation.

State police troopers working in the Norfolk County district attorney’s office never wavered from their judgment that Birchmore’s death was a suicide, according to federal prosecutors. Farwell was interviewed for just 15 minutes in a parking lot, they said. Investigators didn’t take notes, examine him for injuries or sample his DNA. Police did look at Farwell’s phone, but only after he’d deleted his texts with Birchmore.

“Not only did they not investigate, they very quickly told others, including the medical examiner, that this was definitively a suicide and there was no foul play,” Assistant U.S. Attorney Torey Cummings told jurors.