Also, one of the possibilities investigators are looking into is whether gunman Ian David Long believed his former girlfriend would be at the bar, the official said.

Authorities have not determined a motive for Wednesday night's rampage at the Borderline Bar and Grill.

The official was briefed on the investigation but not authorised to discuss it publicly and spoke to The Associated Press on condition of anonymity. The official would not give additional details on what the 28-year-old former Marine posted on his Facebook and Instagram accounts.

A second law enforcement official said that when Long was inside the bar, he apparently stopped shooting and posted to Instagram, based on the time stamps of the posts.

Neither Facebook nor Instagram responded to a request for comment. Long's social media accounts have been taken down.

Long, a former machine gunner who served in Afghanistan, opened fire with a handgun during college night at the bar, then apparently killed himself as scores of police officers closed in.

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".

Several people who knew Long a decade ago as a high-schooler in the suburb of Thousand Oaks said in interviews that he made them uncomfortable, sometimes through aggressive behaviour.

Authorities in Thousand Oaks described an attack of military efficiency. None of those injured was hurt by gunfire, authorities said. Instead, when Long shot, he killed.

Australian Associated Press