A man on trial for killing a teenager 37 years ago knew details of the crime and confessed to his minister, parents and others, leaving law enforcement officials with no doubt he's guilty of murder, a prosecutor said Monday.

Assistant Attorney General Leanne Robbin said there's "no mystery" about who killed 16-year-old Joyce McLain, of East Millinocket, despite the lack of forensic evidence.

Testimony got underway Monday in the trial of Philip Scott Fournier, 57, who was charged in 2016 in the brutal killing in 1980 in East Millinocket. The teenager's body was found two days after she her disappearance, clothed only in her socks and running shoes. She'd been beaten to death.

Her mother, Pamela McLain, told the judge her daughter went for a run after spending the day baby sitting and giving piano lessons on Aug. 8, 1980, the Bangor Daily News reported.

She crossed the street early that evening and turned to say, "See you later, Mom," the mother testified. Her body was later found behind the athletic fields at Schenck High School.

Fournier has been a person of interest in the case from the beginning.

Hours after the teen was last seen, Fournier stole and wrecked a fuel truck, causing a severe head injury that his attorney contends affected his memory.

His statements about Joyce McLain's disappearance and death have been inconsistent over the years because of the skull fracture that left him in a coma, said defense attorney Jon Haddow.

The trial is expected to last several weeks.

Superior Court Justice Ann Murray, who is hearing the testimony without a jury, planned to visit East Millinocket on Monday to view the crime scene.

Law enforcement officials may have zeroed in on Fournier, but the case languished for years.

Fournier was identified as a person of interest in Joyce's death when he was sentenced to prison in 2009 for possession of child pornography.

Before being arrested in March 2016, Fournier was interviewed 26 times. His answers were inconsistent, his lawyer said. But he knew details only the killer could've known, including the fact Joyce McLain was having her period when she died, Robbin said.

Fournier has been jailed since his arrest. If convicted, he faces a prison sentence of 25 years to life. Maine has no death penalty.