The gunman who fatally shot at least 12 people at a crowded country and western dance hall in Thousand Oaks, California, was identified on Thursday as a 28-year-old who had served in the Marine Corps.

The Ventura county sheriff, Geoff Dean, said that the suspect, Ian David Long of Newbury Park, California, apparently took his own life after being confronted by officers responding to the Wednesday night attack. The .45-calibre handgun he used was purchased legally, but had been outfitted with an extended magazine.

The shooting came just over a year after 58 people were killed at a country music festival in Las Vegas when a gunman opened fire from a high-rise hotel room. There was an eerie parallel between the two shootings as some of the same people who emerged from the bar, the Borderline Bar & Grill, described having survived the shooting on the Las Vegas Strip.

Deputies had some interaction with Long the last few years, the sheriff said, including a reported disturbance at his home in April that prompted mental health specialists to talk to him. The health specialists, who discussed with Long his military service and whether he had suffered from post-traumatic stress disorder, determined that he was not an immediate danger to himself or others and that he could not be involuntarily taken to a mental hospital.

Witnesses recalled a chaotic scene at the bar, which was filled with hundreds of people, many of them college students: A gunman opening fire, first at a security guard, as patrons dropped to the dance floor, hid under tables and broke windows to escape.

Sheriff Dean said that at least six off-duty officers were inside when the gunman opened fire. A parent told Sheriff Dean that “they stood in front of my daughter” and protected her. Witnesses also told local media about patrons in the back of the bar who broke out a window and helped a number of people escape.

The number of people wounded in the shooting on Wednesday was unclear, but the Ventura County Sheriff’s Office said about 22 people had been taken to various hospitals. The sheriff, his voice cracking, identified one victim as Sergeant Ron Helus, who was shot when he entered the building.

“He died a hero because he went, he went in to save lives, to save other people’s lives,” Sheriff Dean said.

Country music was playing in the dimly lit bar when people first heard gunshots some time before midnight. Some said they had initially mistaken the sounds for firecrackers.

Sheriff Dean said Long first shot a security officer outside the club and then walked inside, turned to the right and shot “other security and employees” before starting to fire on patrons in the main part of the club. “No idea what the motive was,” the sheriff said. “We don’t believe he was targeting people.”

“I just started hearing these big pops,” said a witness interviewed by a local television station. The gunman also used smoke bombs, said the man, who was not identified, a detail that the sheriff could not confirm.

Then, panic ensued as people tried to flee. “He just kept firing,” the witness said, adding that “people were trying to get out of the window” to run away from the gunman, who had “a big handgun”.

Chyann Worrell, a junior at California State University Channel Islands, said she was at the bar to celebrate the 21st birthday of her friend Nellie Wong for a night of line-dancing with a live DJ. Shortly after 11pm, Worrell said, the gunman, wearing dark clothing and a dark baseball cap, drew his gun. He aimed it at a man near the front of the bar.

Worrell ducked for cover and heard a barrage of bullets. As she ran out of the bar, she said, she saw several bodies sprawled on the floor. Hours after the shooting, she had still not heard from two friends who had been with her at the bar.

One young woman inside the bar, Teylor Whittler, said the gunman appeared focused and did not appear to be targeting anyone in particular.

“I saw him shoot,” Whittler said, adding that someone had yelled, “Everybody get down.”

She said she saw him quickly reload his gun and fire again. “He knew what he was doing,” she said. “He had perfect form.”

“People started running to the back door,” she said, and she heard someone shout, “Get out — he’s coming.” She then fled and heard another burst of gunfire.

Brendan Kelly, 22, helped several people escape from inside. “It’s your worst nightmare,” he said. “It’s terrible.”

Sergeant Helus entered Borderline with an officer in the California Highway Patrol shortly after the first 911 calls, Chief Dean said. The gunfire immediately started shooting at them, striking Sergeant Helus at least several times.