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THOUSAND OAKS, Calif. (FOX 11 / AP) - The gunman who shot and killed 12 people at a country music bar in a Los Angeles suburb was a former military machine gunner who was interviewed by mental health specialists earlier this year after a neighbor reported a disturbance.

Ian David Long, 28, served in the Marines for nearly five years, including a seven-month tour in Afghanistan, according to the Pentagon. He left as a corporal in March 2013 and recently was living with his mother in Thousand Oaks.

In April, a neighbor called authorities to report loud noises coming from Long's house. Deputies found Long "was somewhat irate, acting a little irrationally" and called in a mental health specialist, Ventura County Sheriff Geoff Dean said. That specialist assessed Long but concluded he couldn't be involuntarily committed for psychiatric observation.

"The mental health experts out there cleared him that day," Dean told reporters Thursday, less than 12 hours after the Wednesday night massacre.

They also were concerned that Long might be suffering from post-traumatic stress disorder, Dean said, "basing that (on) the fact that he was a veteran and had been in the Corps."

A next-door neighbor said he called authorities about six months ago when he heard loud banging and shouting come from the ranch-style home where Long and his mother lived.

"I got concerned, so I called the sheriffs," neighbor Tom Hanson said outside his home Thursday as federal and local law enforcement officers searched Long's house. "I was concerned because I knew he had been in the military."

Hanson described Long as an introvert who never caused him problems and that he was "dumbfounded" by the massacre.

Authorities haven't identified what motivated Long to open fire during college night at Borderline Bar & Grill in Thousand Oaks, a city of about 130,000 people about 40 miles (64 kilometers) from downtown Los Angeles.

The dead included 11 people inside the bar and a veteran sheriff's sergeant who was the first officer inside the door. Hundreds fled in terror before Long apparently killed himself.

"Obviously he had something going on in his head that would cause him to do something like this," Dean said.

Court records show Long had been married, but divorce papers were filed in May 2013, soon after he left the Marines.

The Marine Corps said Long earned several awards, including a Combat Action Ribbon and a Marine Corps Good Conduct Medal. He was assigned to the 2nd Battalion, 3rd Marine Regiment, Third Marine Division in Kaneohe Bay, Hawaii.