Ventura County Sheriff's Office Capt. Garo Kuredjian said the first reports of shots fired came around 11:20 p.m.at the Borderline Bar & Grill in Thousand Oaks, which is about 64 kilometers west of Los Angeles.
Paul Delacourt, assistant director in charge of the Los Angeles FBI, told a news conference that it was too early to speculate on the gunman's motives but that he appeared to have acted alone.
"We will be sure to paint a picture of the state of mind of the subject and do our best to identify a motivation", Delacourt said, adding that the FBI would investigate any possible "radicalization" or links to militant groups.
Long was in the Marine Corps from 2008 to 2013, reaching the rank of corporal and serving as a machine gunner in Afghanistan, and the sheriff said he may have suffered from post-traumatic stress disorder.
Helus was pronounced dead at an area hospital early Thursday morning, one of 12 people killed by Long before the gunman took his own life.
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 bad shooting in California".
Deputies were called to Long's house in response to a disturbance in April, 2018 and "felt he might be suffering from PTSD", he said.
Nineteen-year-old Cole Knapp was inside Borderline at the time of the shooting.
"It's a horrific scene in there", Dean said.More news: BioWare Gives Mass Effect Fans A Dash Of Hope For N7 Day
Dean described the scene inside the bar as "like hell". "There is blood everywhere", Ventura County Sheriff Geoff Dean told reporters.
Some people in Five Points told News4Jax that they believe better gun control laws could curb the number of mass shootings.
Only 10 days ago a gunman killed 11 worshipers at a synagogue in Pittsburgh.
Long shot an unarmed security guard outside the bar before going inside, where he shot more security staff, CNN reported.
Trump ordered US flags to be flown at half-staff at public buildings and grounds.
Late Wednesday, the Borderline Bar and Grill was crowded with students who regularly attended its country music dance party.
Those who got out ran, the sounds of rapid gunfire echoing around them in the dark.
"I know that, being a religious person, that God is never going to give me anything more than I can handle", he said.More news: New Fast-Track Measures to Deny Asylum to Illegal Immigrants
"There were people hiding in restrooms, people hiding in attics", he added.
After the shooting was over, Kelly said he and another Marine friend helped victims alongside first responders. The website Niche named it the third-safest city in the United States in 2018.
Speaking through tears, Mr Coffman said: "I talked to him last night, before he headed out the door". "It doesn't matter how safe your community is".
"It can happen anywhere", he said.
He could not immediately provide information on the extent of the victims' injuries.
When the gunshots paused, Wennerstorm said he knew the shooter must be reloading, and he didn't waste the moment. "Oh, son, I love you so much".
Actress Tamera Mowry-Housley confirmed in a statement to ABC News that her niece, Alaina, was killed at the bar.
A former Marine - armed with a Glock 21.45-caliber handgun with an illegal extended magazine - opened fire on a packed California country music bar Wednesday night, killing 12 people, including a veteran sheriff's sergeant nearing retirement, officials said.More news: Europa League: Sarri criticizes Chelsea players after 1-0 win over BATE
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