CONCORD, N.H. -- The stepfather of a man charged with killing his 70-year-old mother while she was in a hospital said the man had a long history of erratic and violent behaviour he blamed on post-traumatic stress disorder from his time in the military.

Bob Ferriere told The Associated Press on Thursday that Travis Frink, of Warwick, Rhode Island, struggled for the past decade with PTSD after he returned from the military with a traumatic brain injury.

He said when Frink was not on his medication he would go "wacko," beating people up and threatening others with a gun, even jumping into a river naked.

"As long as he was taking his medicine for PTSD, he was all right. When he got off the medicine, he would go wacko," Ferriere said in a phone interview. "He beat people up. He would threaten people. He got locked up. He lost his job three or four times because he wouldn't take his medicine."

Frink is charged with fatally shooting his mother, Pamela Ferriere, on Tuesday at Dartmouth-Hitchcock Medical Center in Lebanon, where she was being treated for an aneurysm. He has pleaded not guilty to murder.

Bob Ferriere insisted Frink got along well with his mother and they had a "good relationship." Over the past four years, he said, the couple thought Frink's mental health issues had "been resolved," and a week ago Frink had gone with his three children to visit his mother in the hospital.

"I took them all downstairs in the cafeteria. We had lunch together and had a great time," Ferriere said. "Everything was all hunky-dory until the 12th."

It was a completely different story Tuesday when Frink went to visit her. With Bob Ferriere in the room, Frink asked to spend some time alone with his mother, authorities said, and she said it was OK.

As Bob Ferriere was leaving the room, he heard her shout or scream, then turned and saw Frink pointing a gun at her, according to an affidavit. After several shots were fired, Frink walked out of the room without saying anything, Ferriere told police.

"No one in this God-given world knows why he did it. I can't tell you why he did it," Bob Ferriere said, adding the only possibility was the PTSD, based on his past behaviour.

Ferriere said he believes Frink suffered from PTSD from his time in the Marines and had a traumatic brain injury. But he didn't know when he served nor how he might have gotten injured. He also did not say what medications Frink was taking.

"He had 10 years of fighting PTSD," Ferriere said. "When he didn't take his medicine, he went berserk."

A spokesman for the U.S. Marines did not respond to a request to confirm Frink's military service or his injuries.

Ferriere said he had already forgiven his stepson though he would accept a court's verdict of life in prison or even death for him.