AUBURN – Scott Poirier was depressed, suicidal, abused drugs and alcohol and displayed symptoms of post-traumatic stress disorder, a psychiatrist testified Thursday during the second day of the Sabattus man’s murder trial. But he acted with purpose and knew what he was doing when he killed his father, the expert said.

Dr. Diane Schetky, who was hired by the state to examine Poirier, spent 2½ hours talking with the 35-year-old defendant after poring over police reports and investigating his personal background.

She concluded he was competent to stand trial, then evaluated his likely mental state at the time he fatally shot Roland “Jerry” Poirier at his home during the father’s 65th birthday celebration, she said in Androscoggin County Superior Court.

Schetky said she didn’t think Scott Poirier was psychotic, delusional or suffered from multiple personality disorder when he pulled the trigger of his .270-caliber hunting rifle. On the night of Nov. 8, 2006, a bullet from that rifle traveled through the French doors from outside his parents’ Lewiston home into their dining room and punched a hole in his father’s neck. The bullet had grazed his aunt and shattered a wine bottle on the table, spraying glass on birthday guests.

Schetky said Poirier had been consumed by the memory of waking up as a 15-year-old and finding his father performing a sex act on him.

Earlier on the day of the shooting, Poirier had talked with a family member both about his abuse and about her having being raped when she was 13 by someone outside the family. In the days leading up to the shooting, Poirier and friends had shared stories of sexual abuse, Schetky said.

“You can imagine his rage goes up a couple of notches,” she said. He started drinking mini bottles of liquor that afternoon, building courage to act on his anger, she said. “It’s brewing a very volatile pot.”

Schetky said Poirier’s abuse by a male might have triggered a sexual identity crisis that could have led to his extramarital affair in which he may have been seeking to assert his manhood, she said. That affair led to the collapse of his marriage, she said. That, in turn, fueled his depression.

Asked whether the alcohol interfered with Poirier’s ability to act with the intent of killing his father, Schetky told the prosecutor she couldn’t say that.

For a jury to return a guilty verdict on a murder charge, Assistant Attorney General Lisa Marchese must prove beyond a reasonable doubt that Poirier acted with knowledge or intent when he shot and killed his father.

Schetky has testified in at least 30 homicide cases, including the highly publicized trial of Lee Boyd Malvo, convicted of murder in connection with the 2002 Washington, D.C, sniper slayings.

Under cross-examination by Poirier’s attorney, Steven Peterson, Schetky said Poirier’s preoccupation with suicide could have caused an “abnormal condition of mind,” a legal term used in determining whether a defendant is criminally responsible for his actions.

Peterson will be seeking to show Poirier was not criminally responsible for the death of his father because of his state of mind at the time.

Talking to reporters during a lunch break, Peterson called a statement implicating his client by Poirier’s brother, Raymond, a “total fabrication.” Peterson said the incriminating statement was conveyed to police at a time when he was seeking to buy his brother’s share of the family excavating business. Peterson suggested his client’s brother was hoping Scott Poirier would discount the price to keep Raymond from sharing the statement with police.

Raymond Poirier testified Wednesday that his brother had told him nearly a year before the shooting that he had thought about sighting his father through the scope of a rifle.

A sobbing sister testified Thursday that she had talked to Scott Poirier a half-dozen times during the hours leading up to the shooting. He told her he loved her, but couldn’t take care of her anymore. He also said she needed to look after his four sons.

During his last call, he checked to make sure she wouldn’t be at the party, the sister said. He said he “couldn’t go on anymore. He couldn’t keep living like this.”

His sister told Poirier she needed him, but he said it was too late, then hung up the phone.