Ian David Long’s behavior alarmed roommates and neighbors, and drew police to his home, but never became threatening enough for him to be taken into custody for mental health treatment.

The troubled ex-Marine who stormed into a bar Wednesday night and shot 12 people to death was described as reclusive and agitated, picking fights with teammates in high school and cursing so loudly at his mother that he regularly woke up neighbors. He was killed during the massacre — possibly by his own gun. Authorities said they have found no motive for the killings.

Long, 28, saw combat as a machine-gunner in Afghanistan in 2010, and one of his roommates suspected he had post-traumatic stress disorder.

Though a neighbor in Newbury Park said the fights predated Long’s military service, they grew worse after his discharge in 2013.

Donald J. MacLeod, 79, recalled a night last year when he and his wife were getting ready for bed around 9 p.m. Long and his mother, Colleen, were shouting, and dogs were barking.

“All of sudden I heard a gunshot,” MacLeod said. “I told my wife to leave the lights out, don’t get near the wall, a bullet could go through the stucco.”

He didn’t report the incident. But another neighbor, Tom Hanson, 70, called police in April when it sounded like Long was tearing the house apart. Hanson was worried that Long would hurt himself.

Ventura County Sheriff Geoff Dean said Long was irate and acting irrationally.

The mental health crisis team that was called in to evaluate him discussed that he might have PTSD, based on his military service. They concluded he did not need to be taken into custody.

Dean said Long had several interactions with deputies over the years but was never arrested or involuntarily detained for a mental health evaluation.

Richard Vargas, 29, said he attended El Modena High School in the city of Orange with Long and played on the same freshman basketball and baseball teams.

“The kid was an instigator,” Vargas said. “He was always trying to start fights.”

He said Long insulted inferior players on his teams.

If anyone pushed back, “he’d want to immediately start a fight,” Vargas said.

Around 2006, the family moved to Newbury Park, most of which is part of the city of Thousand Oaks.

Long played reserve outfield on the varsity baseball team during his junior year at Newbury Park High, but mostly kept to himself.

“He was respectful,” said his coach, Scott Drootin. “He wasn’t a very happy kid. I always try to make kids smile, and he never did. He was kind of a loner.”

But at home, neighbors could hear Long shouting at his mother, figuring he was a bratty teenager. His parents had divorced in 1991 when Long was 1, according to Orange County court records.

After graduation in 2008, Long enlisted in the Marine Corps. He married the next year. His mother told neighbors when her son decided to join the military, MacLeod said. “She was proud of him.”

For a few years, things were quiet at the home. MacLeod got to know Colleen Long more. She worked at Procter & Gamble and loved to rescue German shepherds from the shelter. “She has two named Mike and Molly,” MacLeod said.

Ian Long received two administrative disciplinary actions for shoplifting at a store on base before deploying to Afghanistan in 2010 and spending seven months overseas.

He received standard military honors including the Navy Unit Commendation, the Navy Meritorious Unit Commendation, Marine Corps Good Conduct Medal and Combat Action Ribbon, which is given to anyone who is fired on by the enemy or fires at the enemy.

In 2011, he attained the rank of corporal. His last post was at Kaneohe Bay in Hawaii, and he was honorably discharged in 2013. He filed for divorce jointly with his wife that May.

Long enrolled in 2013 at Cal State Northridge, where he studied athletic training.

Blake Winnett, a set builder from Simi Valley, said he lived with Long for about two years in Simi Valley and Reseda.

“He kept to himself, always had his earbuds in,” Winnett recalled. “He went to the gym, went to class or rode his motorcycle.”

Sometimes, Long would go into the Reseda home’s garage and dance alone to electronic dance music, even on sweltering days.

“He’d be in the garage for an hour, 100 degrees outside and in the middle of the day,” Winnett said.

Winnett said he would sometimes coax Long into grabbing a drink at a bar.

Long, he said, occasionally went to Borderline, but the bar’s Western vibe wasn’t really his scene.

The former Marine was known to keep a handgun, but it didn’t strike Winnett, a gun owner, as unusual.

He said Long experimented with molly, a well-known club drug, and took painkillers after a motorcycle accident on a freeway around 2015 that left him with injuries to his hand.

He frequently visited his mother in Newbury Park; the fights resumed as soon as he returned from the service. But they were more frequent, louder and longer than before, sometimes erupting two or three times in a day, with doors slamming and Long shouting profanities at his mother.

“You could barely hear her voice,” MacLeod said. “You could certainly hear him. He was an agitator.”

Another former roommate in the Reseda house, who requested anonymity to protect her privacy, said she felt that Long did have some PTSD. But his personality changed after the motorcycle crash.

“He started taking pills for his pain, and he was just not the same,” she said. “I didn’t know if it was due to the accident itself and the pain, or the pills. He had a character change and was more isolated.”

She said he spent nine months never leaving the house, mostly holed up in his room, before moving back in with his mother in 2016.

“In retrospect, should I have done something?” the roommate said. “I did think to myself, I remember vocalizing this, ‘If I know anyone that might become a shooter, it would be Ian.’ ”

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eric.sondheimer@latimes.com

Times staff writers Joe Mozingo, Hannah Fry, Jaclyn Cosgrove, Javier Panzar, Alejandra Reyes-Velarde, Anh Do and Joe Serna in Los Angeles and Thousand Oaks and David S. Cloud in Washington contributed to this report.