Authorities haven't identified what motivated Long to open fire during college night at Borderline Bar & Grill in Thousand Oaks, around 40 miles (64 kilometres) from downtown Los Angeles.
Ventura County Sheriff Geoff Dean told reporters Thursday that Long had served in Afghanistan. But a mental health professional who spoke with Long said he did not believe Long needed to be hospitalized.
Long had minor run-ins with the law previously, including a traffic accident and a bar fight, law enforcement officials said Thursday.
Nineteen-year-old Cole Knapp was inside Borderline at the time of the shooting.
Knapp, a student at nearby Moorpark College, ducked behind a pool table with a few other people, he said, even as some patrons kept line dancing, apparently unaware of the gunfire around them.
Wednesday night's shooting came less than two weeks after a self-proclaimed anti-Semite was charged with killing 11 people in a Pittsburgh synagogue.
Asked what the scene inside the bar was like, Dean said, "Like. hell". This is a developing story and will be updated as we learn more.
He said Helus was on the SWAT team for much of his career and worked in narcotics and investigations.
Terrified revelers used barstools to break second-floor windows and jump to safety to escape the dance bar, where the gunman was later found dead.
"It's a horrific scene in there", Dean said at a news conference early Thursday in the parking lot of the Borderline Bar & Grill.
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President Donald Trump, who has resisted a surge in calls for tougher gun controls since 17 students were shot dead at Stoneman Douglas High School in Florida earlier this year, said on Twitter that he had been "fully briefed on the awful shooting in California".
"Great bravery shown by police", Trump tweeted.
In a tweet, President Donald Trump applauded the response of police and extended blessings to the victims and their families.
Jerry Nadler, the top Democrat on the House Judiciary Committee, said lawmakers would get to work on legislation including universal background checks when the House of Representatives convenes in January with a Democratic majority.
Tait, who says she's a regular at the bar, then saw people huddled by a mirror and window and fled there. "Maybe the ego got the better of me but it took only one time for a 19-year-old D-2 athlete to talk down to me and tell me how to do my job that I realized this wasn't the career I wanted to head".
"I was confused because we didn't have any balloons".
Long shot an unarmed security guard outside the bar before going inside, where he fired on security staff, CNN reported. "So I wouldn't say I am less like to go out, but definitely more cautious". It was country college night, and people swayed to the beat as music blared through the speakers.
"It makes me think about going out now", King said.
Witness John Hedge told ABC News he was near the front door of the California bar when the shooting began. "There was probably three or four". I hit the ground.
"I would make fun of him, because he would drag me there". It's a reality that Wagner said no bar owner can predict but there are things they can put in place in preparation for such an event. "You just don't expect it to happen in Thousand Oaks".
"I fell to the floor and hid behind him and just tucked myself in his back", she said. "It doesn't matter how safe your community is".