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 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.

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.

Long opened fire, seemingly at random, inside the barn-style, Western-themed bar, with a .45 calibre 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 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.

“He was raving hell in the house, you know, kicking holes in the walls and stuff and one of the neighbours was concerned and called the police,” Richard Berge, who lived one block away from the home, said. 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.