UPDATE: USING a smoke bomb and a handgun, a hooded Marine combat veteran dressed all in black opened fire during university night at a country music bar in Southern California, killing 12 people and sending hundreds fleeing in panic before apparently taking his own life, authorities said.
Police said the motive for the attack was under investigation.
The killer was identified as 28-year-old Ian David Long, a former machine gunner and veteran of the war in Afghanistan who was interviewed by police at his home last April after an episode of agitated behaviour that they were told might be post-traumatic stress disorder.
Screaming in fear, patrons rushed for the exits, ducked under tables and hurled barstools to smash second-floor windows and jump to safety as gunfire erupted at the Borderline Bar & Grill, a hangout popular with students from nearby California Lutheran University.
"I dropped to the floor," Sarah Rose DeSon told Good Morning America.
"A friend yelled, 'Everybody down!' We were hiding behind tables trying to keep ourselves covered."
The dead included 11 people inside the bar and a veteran sheriff's sergeant who was the first officer inside the door, Ventura County Sheriff Geoff Dean said.
"It's a horrific scene in there," Sherrif Dean said in the bar's car park. "There's blood everywhere."
Anxious family members gathered at a nearby community centre in town to await word on the fate of loved ones who had gone to the club.
Jason Coffman received confirmation that his son Cody, 22, who was about to join the Army, was dead. Mr Coffman broke down as he told reporters how his last words to his son were not to drink and drive and that he loved him.
"Oh, Cody, I love you, son," Mr Coffman sobbed.
"I can not believe it's happened to my family," says Jason Coffman, father of California shooting victim Cody Coffman. "I'm speechless and heartbroken.... My life has changed now forever" https://t.co/HVITJqO5xN pic.twitter.com/MPIZONA7O3— CNN Newsroom (@CNNnewsroom) November 8, 2018
Authorities undertook a search of Long's home in Newbury Park, about eight kilometres from the Borderline bar, for clues to what set him off.
"There's no indication that he targeted the employees. We haven't found any correlation," the sheriff said. "Maybe there was a motive for this particular night, but we have no information leading to that at all." Long was in the Marines from 2008 to 2013, rose to the rank of corporal and served in Afghanistan in 2010-11, the military said.
According to the New York Post, Sheriff Dean said his officers had several interactions with Long, including an April call to his home.
"We've had several contacts with Mr. Long over the years, minor events, a traffic collision," he said.
"In April of this year, deputies were called to his house for a subject disturbing. They went to the house, they talked to him. He was somewhat irate. Acting a little irrationally."
Mental health professionals sent to evaluate him concluded he did not need to be taken into custody, he said.
Long also was the victim of an assault at a local bar in 2015, the sheriff said.
The sheriff also said there were six off-duty police officers from various agencies at the Borderline at the time of the attack.
The country-themed venue was filled with university students when Long tossed a smoke bomb and opened fire at around 11.20pm local time (6.20pm AEDT).
Long purchased the gun in Ventura County, according to the Los Angeles Times.
The identification process took longer than usual because the suspect wasn't carrying a driver's license or ID at the time, NBC News said.
Long's motive was unclear, but Sheriff Geoff Dean said there was no evidence the slaughter was linked to terrorism.
US President Donald Trump took to Twitter to offer condolences after the shooting.
Mr Trump said on Twitter that he had been "fully briefed on the terrible shooting."
He praised police, saying "Great bravery shown by police" and said "God bless all of the victims and families of the victims."
I have been fully briefed on the terrible shooting in California. Law Enforcement and First Responders, together with the FBI, are on scene. 13 people, at this time, have been reported dead. Likewise, the shooter is dead, along with the first police officer to enter the bar....— Donald J. Trump (@realDonaldTrump) November 8, 2018
....Great bravery shown by police. California Highway Patrol was on scene within 3 minutes, with first officer to enter shot numerous times. That Sheriff’s Sergeant died in the hospital. God bless all of the victims and families of the victims. Thank you to Law Enforcement.— Donald J. Trump (@realDonaldTrump) November 8, 2018
Patrons screamed in fear, shouted "Get down!" and used barstools to smash second-floor windows and jump to safety as gunfire erupted at the bar, a popular hangout for uni students.
The dead included 11 people inside the bar and a sheriff's sergeant who was the first policeman inside the door, Ventura County Sheriff Geoff Dean said.
"It's a horrific scene in there," Sheriff Dean said in the bar's car park. "There's blood everywhere."
Survivors of the shooting in described moments of panic and heroism as the gunman turned a dance floor into a killing zone.
Nineteen-year-old Tayler Whitler says she was dancing and her friends were at a table by the door as the gunman opened fire.
Ms Whitler said she heard people yell, "get down!", and then silence for a couple seconds, before she heard someone say, "get up, he's coming!".
Ms Whitler also said "people were trampling on each other to get out".
Nineteen-year-old Erika Sigman found herself hiding with a group of strangers, holding her hands.
"There's a lot of bad in this world, but there is also a lot of good and people will help," Ms Sigman said.
Other people described jumping out of windows, piling on top of each other in the bathrooms or hiding under pool tables.
A sobbing girl at the scene said she and about 15 others used bars stools to break windows and get out.
"Borderline's a family," she said, referring to the popular bar. "We all know each other, we're all friends."
Another survivor told local media Long was wearing a black hoodie and holding a handgun when he opened fire.
#BREAKING: Thousand Oaks mayor confirm to local media outlet KTLA that mass shooting suspect was "engaged and neutralized" by police (Video: KTLA)— Matthew Keys (@MatthewKeysLive) November 8, 2018
Live coverage: https://t.co/lIyYNsJwSq pic.twitter.com/TRS15pfkAU
Uni student Cole Knapp, who attends the nearby Moorpark College, was inside the bar when the shooting began.
Mr Knapp said he yelled for people to take cover and fled through an exit door to a closed patio where he told people "everybody get over the fence as quickly as you can," and followed them over.
#THOUSANDOAKS UPDATE: Authorities said at least six people have been wounded, including a sheriff’s deputy, in the mass shooting at #Borderline. The suspect may also still be on the loose. https://t.co/3gvXBtKg7t pic.twitter.com/zIK11Ltsj6— ABC7 Eyewitness News (@ABC7) November 8, 2018
Mr Knapp said he found a highway patrol officer nearby who just happened to be pulling someone over, and he screamed to him, "There's a shooter in there!"
Mr Knapp described the officer as being "kind of in disbelief," then saw he was serious. The student revealed he has friends who haven't been accounted for.
The massacre was the deadliest mass shooting in the US since 17 students and teachers were slain at a Parkland, Florida, high school nine months ago.
It also came less than two weeks after a gunman killed 11 people at a synagogue in Pittsburgh. That, it turn, closely followed the series of pipe bombs mailed to critics of President Donald Trump.
EARLIER: AUTHORITIES say at least 11 people including a sheriff's deputy were shot after a gunman opened fire at a popular bar in Southern California.
Police told the Los Angeles Times that at least 30 shots were fired at the Borderline Bar & Grill in Thousand Oaks on Wednesday night (local time), which is about 40 miles west of Los Angeles.
According to Fox News the club was hosting a college night.
ABC 7 is reporting some of the people who were at Borderline were also at the Route 91 Harvest Festival in LA, the scene of the worst mass shooting in modern US history.
Ventura County Sheriff's Office Capt. Garo Kuredjian said at a news conference early Thursday that the sheriff's deputy was taken to a hospital.
He could not immediately provide information on the extent of the victims' injuries.
The sheriff's spokesman said the responding deputies could hear gunshots as they arrived on scene.
Authorities are still treating it as an active shooter scene.
Officers at the scene indicated that soon after the gunfire erupted, victims were running to a nearby gas station to get medical help.
The initial report at 11.20pm local time indicated that a man had opened fire with a semi-automatic gun.
Law enforcement and emergency crews were flooding the scene - including the FBI and bomb squad.
Live video posted by KABC-TV of Los Angeles showed aerial footage of armed officers entering the nightclub and numerous police vehicles outside.
As officers were responding, there were additional reports of gunfire.
A witness told ABC7: "I was at the front door talking to my stepdad. I heard these big pops.
"There were three or four and I hit the ground. The security guard was dead and the man had a handgun.
"The gunman had glasses and a black jacket. He had a big hand gun."
Another witness said: "This maniac came in. Threw in smoke to confuse people and opened fire on the dancefloor. He's taken many young lives".
Authorities urged the public to avoid the area.
Borderline Bar & Grill originally opened in Malibu in the 1980s, but moved to Thousand Oaks in 1993, according to its website.
The bar is described as the county's "Largest Country Dance Hall & Live Music Venue" and boasts 2500 square feet of open dance floor.