THOUSAND OAKS, Calif. — Using a smoke bomb and a handgun, a hooded former Marine dressed all in black opened fire during college 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 Thursday.

Authorities said the motive for the attack Wednesday night was under investigation.

The killer was identified as Ian David Long, a 28-year-old veteran who authorities said had an episode of erratic behaviour last spring that was thought to be post-traumatic stress disorder because of his military background.

Patrons at the bar screamed in fear, shouted “Get down!” and used 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. The dead included 11 people inside the bar and a sheriff’s sergeant who was the first officer inside the door, Ventura County Sheriff Geoff Dean said.

“It’s a horrific scene in there,” Dean said in the parking lot. “There’s blood everywhere.”

The killer deployed a smoke device and used a .45-calibre handgun, a law enforcement official told The Associated Press. The official was not authorized to discuss the investigation publicly and spoke on condition of anonymity.

It was the deadliest mass shooting in the U.S. since 17 students and teachers were killed at a Parkland, Florida, high school nine months ago. It also came less than two weeks after a gunman massacred 11 people at a synagogue in Pittsburgh. That, in turn, closely followed the series of pipe bombs mailed to critics of President Donald Trump.

Trump praised police for their “great bravery” in the California attack and said, “God bless all of the victims and families of the victims.” He ordered flags flown at half-staff in honour of the victims.

Dean said his department had several previous contacts with Long, including a call to his home in April, when deputies found him angry and acting irrationally because of what authorities said might have been PTSD. The sheriff said a mental health crisis team was called at the time and concluded Long did not need to be taken into custody.

Dean said the other prior encounters were a traffic accident and an incident in which Long was the victim of a battery at a bar.

As for why he attacked the bar, “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.”

The gunman was tall and wearing all black with a hood and his face partly covered, witnesses told TV stations. He first shot a security guard standing outside, then went in and opened fire inside the nightclub, the sheriff said.

“I dropped to the floor,” Sarah Rose DeSon told ABC’s “Good Morning America.” ”A friend yelled, ‘Everybody down!’ We were hiding behind tables trying to keep ourselves covered.”

Sheriff’s Sgt. Ron Helus and a passing highway patrolman arrived at the Borderline around 11:20 p.m. in response to several 911 calls, heard gunfire and went inside, the sheriff said.

Helus was immediately hit with multiple gunshots, Dean said. The highway patrolman pulled Helus out, then waited as a SWAT team and scores more officers arrived. Helus died early Thursday at a hospital.

By the time they entered the bar again, the gunfire had stopped, according to the sheriff. They found 12 people dead inside, including the gunman, who was discovered in office and had apparently shot himself, the sheriff said.

“There’s no doubt that they saved lives by going in there and engaging with the suspect,” said Dean, who was set to retire on Friday. He praised the slain officer — a close friend — as a hero: “He went in there to save people and paid the ultimate price.”

Shootings of any kind are extremely rare in Thousand Oaks, a city of about 130,000 people about 40 miles (64 kilometres) west of Los Angeles, just across the county line.

The Borderline, which includes a large dance hall along with several smaller areas for eating and drinking, was holding its regular “College Country Nights” Wednesday night when the attack took place.

The bar is also close to several other universities, including California State University Channel Islands in Camarillo, Pepperdine University in Malibu and Moorpark College in Moorpark.

Nick Steinwender, Cal Lutheran student body president, told KTLA-TV he immediately started receiving messages about the shooting, and he and his roommate went to the scene to offer rides back to campus or moral support.

“It’s going to be a very sombre day,” Steinwender said. “I know we don’t have all the details in yet, but you know, it just feels like it’s an attack on our community. You know, I think it’s going to be something that we’re going to have to come together and move past.”

When the gunman entered, people screamed and fled to all corners of the bar, and a few threw barstools through the windows and helped dozens to escape, witnesses said.

Video accessed by the AP showed law enforcement officers and vehicles speeding to the scene and people running from the bar. Rapid-fire gunshots could be heard as officers crouched behind a police vehicle, weapons drawn. Three people were seen carrying someone, and paramedics applied bandages to the man, who had blood on his back.

Cole Knapp, a freshman at Moorpark College, said he was inside the bar when the shooting began, but he thought at first that it was “just someone with an M-80, just kind of playing a prank.” Then he said he saw the gunman, wearing a small black head covering and black hoodie and holding a handgun.

“I tried to get as many people to cover as I could,” Knapp said. “There was an exit right next to me, so I went through that. That exit leads to a patio where people smoke. People out there didn’t really know what was going on. There’s a fence right there so I said, ‘Everyone get over the fence as quickly as you can,’ and I followed them over.”

He said a highway patrol officer who happened to be pulling someone over was nearby.

“I screamed to him, ‘There’s a shooter in there!’ He was kind of in disbelief, then saw that I was serious,” Knapp said. He said he had friends who hadn’t been accounted for.

Tayler Whitler, 19, said she was on the dance floor with her friends nearby when she saw the gunman shooting and heard screams of “Get down!”

“It was really, really, really shocking,” Whitler told KABC-TV as she stood with her father in the parking lot. “It looked like he knew what he was doing.”

The slain sheriff’s officer was a 29-year veteran of the force with a wife and son and planned to retire in the coming year, said the sheriff, who choked back tears as he talked about the sergeant who was also his longtime friend.

“Ron was a hardworking, dedicated sheriff’s sergeant who was totally committed,” Dean said, “and tonight, as I told his wife, he died a hero because he went in to save lives.”

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AP journalists Andrew Dalton in Los Angeles, Michelle A. Monroe in Phoenix and Michael Balsamo in Washington contributed to this report.

Krysta Fauria, The Associated Press