Long served in the Marine Corps from August 2008 to March 2013, with a deployment in Afghanistan between November 2010 and June 2011, military records show.
They believe he may have taken his own life but have not confirmed this.
The sheriff's department had several previous contacts with Long including a call to his home in April when deputies found him acting irate and irrationally.
The police called in a team of mental health specialists who decided that Long did not qualify to be involuntarily detained for mental health evaluation under state law.
The sheriff presiding over the investigation into a mass killing Thursday in Thousand Oaks, California, recalled his visit to a synagogue following last month's synagogue massacre in Pittsburgh.
Dressed in black, the shooter is still "active" and has exchanged fire with cops who were alerted to the attack at 11.20pm local time. The identities of the other victims have not been released at this time.
Survivors described moments of panic and heroism.
Pepperdine officials say they're offering support to students through the university's counseling center and chaplain's office.
Authorities say they found a handgun at the scene.
Zoe Walsh also said that she's angry and heartbroken, and that she's feeling her friend's absence every second.
"For many of us, it was seeing each others' faces in the bar again that brought us a huge sense of relief", she said in an interview with HuffPost.
Teen 'kills mum after arguing over bad school grade'
Chitwood said the teen left school early on Friday to stage a burglary at the home where he killed his mother, police said. Police say Ramos and his mother got into a awful argument after he came home with a "D" in one of his subjects.
Erika Sigman, 19, was another witness.
"I know that, being a religious person, that God is never going to give me anything more than I can handle", he said.
Housley's roommate said that she's devastated, but also angry.
KNBC-TV spoke to a man at a reunification center which had been set up for families. "One bar stool and straight through a window", he told reporters.
The bar had been hosting a country music night for university students when the gunman opened fire.
The Borderline Bar shooting left 12 dead, including the police officer who responded to first reports.
"I've learned it doesn't matter what community you're in", Dean told reporters when asked if he was surprised this happened in Thousand Oaks. Armed with a knife in case an attacker came in, he hunkered down and waited with 40 other people for four hours.
Matt Wennerstron, a 20 year old college student and regular at the bar, said the shooter fired a short-barreled pistol that apparently had a 10-15 round magazine. Dean said Helus was the first to go in and was immediately hit with multiple gunshots. Helus and a California Highway Patrol officer were the first to arrive at the bar and went inside just before 11:30 p.m. PST (0730 GMT).
The gunman threw smoke bombs when he entered, perhaps to cause confusion, CBS L.A. reported.
"It's got to change", said Gavin Newsom, governor-elect of California, at a news briefing on Thursday. The bar is near the campus.