Police said the gunman was found dead inside the bar on the outskirts of Los Angeles although it was not immediately clear if he was killed by officers or shot himself.
Speaking at press conference in the wee hours of Thursday, a sheriff said that around a dozen other people had been injured. He said the motive of the shooting and the identity of the shooter were not known.
It was the second mass shooting in America in less than two weeks.
Witnesses said that the gunman, who was wearing a black trenchcoat, throw several smoke grenades inside the Borderline Bar and Grill before he started he shooting at around 11:20pm on Wednesday night.
"It's a horrific scene in there. There is blood everywhere," Ventura County Sheriff Geoff Dean told reporters.
"We have no idea if there is a terrorism link to this or not. As you know, these are ongoing investigations and that information will come out as soon as we are able to determine exactly who the suspect was and what motive he might have had for this horrific event."
"Nothing has led me to believe or the FBI there is a terrorism link here. We certainly will look at that option."
Dean said that the dead police officer, who was named as Ron Helus and had been on the force for 29 years, was among the first on the scene.
"They found 11 victims that had been killed," said Dean of the first response unit before detailing that the death of Helus brought the toll to 12, not including the gunman.
The venue in the quiet, upscale Thousand Oaks suburb had been hosting an event for college students, with possibly several hundred young people in attendance, Captain Garo Kuredjian of the Ventura County Sheriff's office said earlier.
A witness talks to reporters outside the bar, which was packed with students on a so-called college night. Photo by AFP |
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.
"It was just semi-automatic, as many shots as he could pull, and then when it started to reload that's when we got people out of there and I didn't look back."
He said he and others smashed their way out of the bar onto a balcony and then jumped down to safety. "One bar stool and straight through a window," he told reporters.
TV footage showed SWAT teams surrounding the bar, with distraught revelers milling around and using their cell phones as lights from police cars flashed.
Holden Harrah, a young man who saw the incident, cried as he told CNN that a place where he goes every week to have fun with friends had been a scene of carnage.
"A gentleman walked in the front door and shot the girl that was behind the counter. I don't know if she is alive," he said.
'He shot a lot'
The Los Angeles Times quoted a law enforcement official as saying at least 30 shots had been fired.
An unnamed witness told the newspaper that someone ran into the bar around 11:30 pm and started shooting what looked to be a black pistol.
"He shot a lot, at least 30 times. I could still hear gunshots after everyone left," the Times quoted the man as saying.
It was the latest chapter in America's epidemic of gun violence.
Only 10 days ago a gunman killed 11 worshipers at a synagogue in Pittsburgh.
That shooting was politically sensitive: the suspect, Robert Bowers, who said he wanted to kill Jews, argued that a Jewish advocacy group had been aiding a Central American migrant caravan denounced repeatedly by President Donald Trump in the run-up to Tuesday's midterm election.
Last year a country music festival called Route 91 in Las Vegas was the scene of the worst mass shooting in modern US history. A gunman shooting from the 32 floor of a hotel and casino with high power weapons killed 58 people.
Carl Edgar, a 24 year old regular at the Thousand Oaks club, said he was in the bar with about 20 friends and had not been able to reach some of them since the shooting. They may have turned their phones off, he said.
"A lot of my friends survived Route 91," he told the Times. "If they survived that, they will survive this."