Law enforcement has identified Ian David Long as the shooter in Wednesday's deadly shooting near Los Angeles.
The 28-year-old former Marine had several prior contacts with authorities, including during an incident in April when he was reportedly acting erratically. Mental health experts were dispatched at that time but cleared him, investigators say.
He has also been a crime victim. Detectives say he was once the victim of a battery at a California bar.
Authorities are working on getting a search warrant for his home in Newbury Park. They are also working to determine if he was living with Post-Traumatic Stress Disorder.
On Wednesday night, Long drove his mom's vehicle to the bar, according to the Los Angeles Times.
He was reportedly hooded and dressed entirely in black.
Authorities say he used a .45-caliber Glock 21 handgun during the bar attack. The weapon was purchased legally. Investigators say it had an extended chamber to hold more rounds.
Investigators don't believe the shooter said anything as he opened fire.
PHOTOS: Deadly California bar shooting
According to the Los Angeles Times, at least 30 shots were fired.
Long's body was found inside an office next to the bar entrance.
Hundreds of people ran from the building. Some even broke windows and jumped out to escape the massacre.
Twelve victims, including a sheriff's deputy, are confirmed dead. The gunman is also dead.
The fallen law enforcement officer, who was killed while trying to stop the shooter, has been identified as Sgt. Ron Helus of the Ventura County Sheriff's Office.
As of late Thursday morning, investigators believe the shooting was a random attack.
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