The gunman, 28-year-old Ian David Long, entered the Borderline Bar and Grill in Thousand Oaks, a suburb 40 miles (64km) northwest of downtown Los Angeles, and opened fire at a little before midnight before he apparently took his own life
The FBI is hoping to build a clear profile of a former US Marine combat veteran who killed 12 people in a crowded Los Angeles area bar to discover a motive for the latest shooting massacre in the United States.
The gunman, 28-year-old Ian David Long, entered the Borderline Bar and Grill in Thousand Oaks, a suburb 40 miles (64km) northwest of downtown Los Angeles, and opened fire at a little before midnight before he apparently took his own life, law enforcement officials said.
The massacre was the latest shooting rampage in the United States amid a fierce debate over gun control, coming less than two weeks after a man shot dead 11 worshippers at a Pittsburgh synagogue.
Paul Delacourt, assistant director in charge of the Los Angeles office of the FBI, said it was too early to speculate on the shooter's motives but that he appeared to have acted alone.
"We will be sure to paint a picture of the state of mind of the subject and do our best to identify a motivation," Delacourt said, adding that the FBI would investigate any possible "radicalization" or links to militant groups.
Long opened fire, seemingly at random, inside the barn-style, Western-themed bar, with a .45 caliber Glock handgun equipped with a high-capacity magazine, Ventura County Sheriff Geoff Dean said. The bar was packed with college students.
Long was in the Marine Corps from 2008 to 2013, reaching the rank of corporal and serving as a machine gunner in Afghanistan, and the sheriff said he may have suffered from post-traumatic stress disorder.
"Obviously, he had something going on in his head that would cause him to do something like this," Dean said.
Dean told reporters that in April officers had gone to Long's home in nearby Newbury Park, about 4 miles (6km) from the bar to answer a disturbance call and found him agitated. Mental health specialists talked with Long and determined that no further action was necessary, the sheriff said.
"He was raving hell in the house, you know, kicking holes in the walls and stuff and one of the neighbors was concerned and called the police," Richard Berge, who lived one block away from the home, told Reuters.
Berge, who took care of Long’s mother’s dogs, said she told him following that incident she worried her son might take his own life but did not fear he would hurt her.
Dean said he had been told that 150 to 200 people were inside the bar at the time of the shooting.
Asked what the scene inside the bar was like, Dean said, "Like ... hell." Earlier he had described it as "a horrific scene in there. There is blood everywhere and the suspect is part of that."
The Ventura County Sheriff's Department said 21 people had been treated for injuries and released at area hospitals.
Ventura County Sheriff's Office Sergeant Ron Helus, a 29-year veteran, was killed during the shooting. He and a California Highway Patrol officer were the first to arrive at the bar to confront the gunman.
Thousand Oaks, a leafy, sprawling suburb of 127,000 people, was named the third safest city in the United States for 2018 by the Niche research company.
Jason Coffman wept as he told reporters that his son, Cody, 22, was among the dead.
"I know how I love, how much I miss him," he said. "Oh, son, I love you so much."