The gunman who 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 months ago after a neighbor reported a disturbance.

This 2017 photo from the California Department of Motor Vehicles shows Ian David Long. Authorities said the Marine combat veteran opened fire Wednesday evening, Nov. 7, 2018, at a country music bar in Southern California, killing multiple people before apparently taking his own life.[Photo:AP]

Ian David Long, 28, apparently killed himself after the Wednesday attack. He had joined the Marines and gotten married young. Within several years, he left the military and divorced. Later, he enrolled at a university and most recently lived with his mother in a home where neighbors said they could hear aggressive arguments.

Long's service began when he was 18 and lasted nearly five years, including a seven-month tour in Afghanistan, according to the Pentagon. He was honorably discharged with the rank of corporal in 2013.

In April, a neighbor called authorities to report loud noises coming from the house that Long shared with his mother in a Thousand Oaks neighborhood of well-manicured lawns and homes.

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."

People cry as a law enforcement motorcade escorts the body of Ventura County Sheriff's Department Sgt. Ron Helus from the Los Robles Regional Medical Center Thursday, Nov. 8, 2018, in Thousand Oaks, Calif., after a gunman opened fire Wednesday evening inside a country music bar, killing multiple people including Helus.[Photo:AP]

Long's only other contact with authorities came after a traffic collision and after he alleged he was the victim of a violent encounter in 2015 at another bar in Thousand Oaks, Dean said.

A next-door neighbor said he called authorities about six months ago when he heard loud banging and shouting come from Long's ranch-style home.

"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, where an American flag flew over the garage. "I was concerned because I knew he had been in the military."

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

Another neighbor described frequent, aggressive shouting between Long and his mother, especially over the last year. About 18 months ago, Don and Effie Macleod heard "an awful argument" and what he believes was a gunshot from the Longs' property. Don Macleod said he did not call police but avoided speaking with Ian Long.

Los Angeles County Deputy Sheriff Armando Viera, center, consoles an unidentified woman on a freeway overpass after a motorcade with the body of Ventura County Sheriff's Sgt. Ron Helus went by Thursday, Nov. 8, 2018, in Newbury Park, Calif. Helus was fatally shot while responding to a mass shooting at a country music bar in Southern California. [Photo：AP]

"I told my wife, 'Just be polite to him. If he talks, just acknowledge him, don't go into conversation with him,'" Don Macleod said Thursday.

Authorities haven't identified what motivated Long to open fire during college night at Borderline Bar & Grill in Thousand Oaks, around 40 miles (64 kilometers) from downtown Los Angeles. The city of about 130,000 people is consistently near the top of lists of the safest places in California.

The dead included 11 people inside the bar and a veteran sheriff's sergeant who was the first officer through the door.

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

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.

Long's mother, Colleen, posted Facebook photos of her son in his military uniform in 2010 and 2011.

"My Son is home, well sort of, back in Hawaii, soon to be in Cali come January, hooray!" she wrote on Dec. 14, 2012.

Rebecca Raede holds a sign reading "We are T.O. Strong" across the street from the Thousand Oaks Teen Center, where relatives and friends gathered in the aftermath of a mass shooting, Thursday, Nov. 8, 2018, in Thousand Oaks, Calif. Multiple people were shot and killed late Wednesday by a gunman who opened fire at the Borderline Bar & Grill, which was holding a weekly country music dance night for college students. [Photo：AP]

Another photo from 2014 shows Ian Long with his arm draped around his mother in front of Dodger Stadium. The two were wearing Dodgers T-shirts and smiles.

Court records show Long was married as a 19-year-old in Honolulu in June 2009, and he and his wife separated in June 2011 while he was deployed to Afghanistan. The couple cited irreconcilable differences in divorce papers filed in May 2013, two months after Long left the Marines. Their marriage officially ended that November.

California State University, Northridge, said in a statement that Long was a student, last attending in 2016, but offered no further details.