The suspect had served in the U.S. Marine Corps, according to Ventura County Sheriff Geoff Dean.
The man suspected of opening fire in a crowded California bar has been identified as Ian Long.
Law enforcement sources told ABC News that it appeared Long, who was in his late 20s, used a legally purchased .45-caliber Glock in the massacre at Borderline Bar & Grill in Thousand Oaks Wednesday night.
Long lived close to the bar in question, officials said. He was found dead inside the bar of an apparent self-inflicted gunshot wound.
Ventura County Sheriff Geoff Dean said in a press conference Thursday morning that the gunman was a veteran, having served in the U.S. Marine Corps.
Long used a handgun with an "extended magazine" on it, Dean said. It's unclear whether he reloaded the weapon after initially opening fire.
"We believe that's the only weapon that was used, was that handgun," Dean said.
According to the sheriff, the gunman forced his way into the bar after shooting a security guard outside. He then stepped inside and shot more security guards and bar employees just by the door, before firing at the crowd.
"Obviously he had something going on in his head that would cause him to do something like this," Dean said. "Obviously he had some sort of issues."
Dean said it did not appear as though Long was targeting specific people.
Long had previously been the victim of a battery at another bar three years ago, Dean said.
Twelve people were killed, including Ventura County Sgt. Ron Helus, who died responding to the shooting.
Authorities said early Thursday that they were still working to determine a motive.
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