Authorities trying to make sense of why a gunman killed 12 people at a Southern California bar are not publicly discussing what they've learned, but at least one Instagram post he made after beginning the massacre has emerged as an early focus.

Social media platforms have scrubbed that and any other posts following Wednesday night's massacre. But one law enforcement official said Ian David Long, a 28-year-old former Marine, posted about his mental state and whether people would believe he was sane.

Authorities also were investigating whether he believed his former girlfriend would be at the Borderline Bar and Grill, said the official, who was briefed on the investigation but not authorized to discuss it publicly and spoke to The Associated Press on condition of anonymity.

A second law enforcement official, Ventura County Sheriff's Capt. Garo Kuredjian, said that — based on time stamps — the gunman apparently stopped shooting inside the bar and posted to Instagram. Kuredjian said he didn't know the content of any posts. Instagram and Facebook typically refuse to discuss individual accounts and did not respond to a request for comment.

Authorities described an attack of military efficiency. None of those injured was hurt by gunfire. When the gunman shot his .45-caliber pistol, he killed. As scores of police officers closed in, Long apparently shot and killed himself.

Several people who knew Long in the suburb of Thousand Oaks where the gunman went to high school and eventually moved back in with his mother described him in disturbing terms.

Long made others feel uncomfortable going back to his teens.

Dominique Colell, who coached girls' track and field at the high school where Long was a sprinter, remembers an angry young man who could be verbally and physically combative.

In one instance, Colell said Long used his fingers to mimic shooting her in the back of the head as she talked to another athlete. In another, he grabbed her rear and midsection after she refused to return a cellphone he said was his.

"I literally feared for myself around him," Colell said in an interview Friday. "He was the only athlete that I was scared of."

Colell said she wanted to kick Long off the team, but the boy's coach urged her to reconsider because that could compromise his goal of joining the Marines. She relented when, at the next track meet, Long apologized in front of several coaches and administrators.

Attempts to get comment by phone and in person from officials at Newbury Park High School and its school district were unsuccessful. Both were closed because of a destructive wildfire in the area.

As investigators worked to figure out what set him off, President Donald Trump blamed mental illness, describing the gunman as "a very sick puppy" who had "a lot of problems." At the White House on Friday, Trump touted his efforts to fund work on post-traumatic stress disorder among veterans and ignored questions about stricter gun control laws.

Investigators have not commented on whether mental illness played a role in the rampage. But a mental health specialist who assessed Long after sheriff's deputies responded to a call about his agitated behavior last spring worried he might be suffering from PTSD.

The incident happened in April, when yelling and loud banging noises coming from the home Long shared with his mother prompted a next-door neighbor to call authorities. The mental health specialist concluded there were no grounds to have him involuntarily committed.

Among the dead in the shooting rampage were a sheriff's sergeant gunned down as he entered the bar and a U.S. Navy veteran who survived last year's massacre in which a gunman in a high-rise Las Vegas hotel killed 58 people at an outdoor country music festival.