PORTLAND – Jurors in a murder trial that got under way Monday must determine whether a Casco man knew his small-caliber revolver was loaded when he shot his longtime girlfriend in the head.

Gary Sweeney admitted to police that he shot and killed Christine Pepin after a night of drinking and fighting on Feb 4. But he said he didn’t think there were any bullets in the gun.

Police initially charged Sweeney with manslaughter, but upgraded the charge to murder, which carries a penalty of 25 years to life in prison, after additional evidence came to light.

Several witnesses testified Monday in Cumberland County Superior Court that Sweeney had admitted pulling the trigger. But some also testified that he said the slaying was an accident.

The couple’s 12-year-old son took the stand, testifying that he had been with his parents at a neighbor’s home the night of the shooting.

The boy said a snowball fight between himself and his father got out of hand, and Sweeney got mad. The 12-year-old testified that he heard his father ask “Where’s my bullets?” and heard his mother crying, which prompted him to run to a neighbor’s home.

Sweeney soon followed, walking down the road, blood on his clothes and hands, yelling for help. The boy told police he heard his father say, “She’s dead,” “I can’t believe I did it,” “How did that bullet get in there?” and “I’m sorry.”

Prosecutors must prove that Sweeney knowingly and intentionally killed Pepin, while the defense will try to show that it was an accident.