AUBURN – A man facing a murder charge in the fatal shooting of his father is expected to change his plea in court today to a psychiatric defense.

That means Scott Poirier, 34, of Sabattus, could end up saying he is not guilty because he wasn’t criminally responsible by reason of insanity. If successful, he would be committed to a mental institution.

Poirier also could claim he had an “abnormal condition of mind” during the shooting. If a jury agreed, he would be set free.

A third psychiatric defense option would reduce the charge from murder to manslaughter.

Poirier was charged in the Nov. 8 slaying of his father, Roland Poirier, at the family home at 302 Grove St., Lewiston.

Androscoggin County Superior Court Justice Ellen Gorman ordered psychological evaluations of Poirier in December at the request of prosecutors. The results of those examinations have remained sealed, but should be released to Poirier’s attorney as well as state prosecutors today if Poirier were to enter his expected plea change, said his Waterville attorney, M. Michaela Murphy.

She said Poirier could have asserted a psychiatric defense at the time he was indicted on the murder charge, but he had no history of ongoing psychosis or significant mental health problems, other than depression. His lawyers conducted their own investigation and psychological exams before deciding with their client to move forward with such a defense, Murphy said.

“It’s a significant decision,” she said. “It’s not one you make lightly.”

Poirier already entered a plea of “not guilty” to the murder charge. He is being held without bail.

Police said Roland Poirier was shot in the back of the neck as he sat at a table celebrating his 65th birthday with a roomful of family and friends.

According to police reports, Scott Poirier stood outside in the yard with a rifle and fired a gunshot through a sliding glass door, killing his father.

Police said he claimed he shot his father because he and others had been sexually abused by the man for years when he was younger.