CALIFORNIA, USA (AP) — Using a smoke bomb and a handgun, a hooded Marine combat veteran 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 yesterday.

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

The killer was identified as 28-year-old Ian David Long, a former machine gunner and decorated veteran of the war in Afghanistan who was interviewed by police at his home last spring 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 ABC’s 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,” Dean said in the parking lot. “There’s blood everywhere.”

The bloodshed was the latest in what seems to be a never-ending string of mass shootings that are happening with terrifying frequency across the United States.

It was the nation’s deadliest such attack 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.

President Donald 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.

Long was armed with a Glock 21, .45-calibre, designed to hold 10 rounds plus one in the chamber, according to the sheriff. But it had an extended magazine — one capable of holding more ammunition — that is illegal in California, Dean said.

The killer also deployed a smoke device, a law enforcement official told The Associated Press. The official was not authorised to discuss the investigation publicly and spoke on condition of anonymity.

Authorities undertook a search of Long’s home in Newbury Park, about 5 miles 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.”

The Marine Corps said Long served from 2008 to 2013, rose to the rank of corporal and was sent to Afghanistan in 2010-11, receiving several medals and commendations.

Authorities said he had no criminal record, but Dean said officers were called to his home in April, when deputies found him angry and acting irrationally. The sheriff said officers were told he might have PTSD because of his military service. A mental health specialist met with him and didn’t feel he needed to be hospitalised.

Tom Hanson, 70, who lives next door to Long and his mother, said Wednesday that he called the police about six months ago when he heard “heavy-duty banging” and shouting coming from the Longs’ home.

“I was concerned because I knew he had been in the military,” he said.

Hanson said the sheriff’s deputy who arrived took his information, but he never learned more about what happened and hadn’t spoken to Long since then. He said he was “dumbfounded” by the bloodshed.

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 on staff members and patrons, the sheriff said.

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