Ian David Long opened fire with his handgun after storming the Borderline Bar & Grill In Thousand Oaks. He first shot a group of security guards and employees near the entrance before aiming at victims on the dance floor.
The 28-year-old former Marine’s Instagram account was still active several hours after he had apparently killed himself, but was immediately shut down after authorities became aware of it's existence.
He had also posted messages about being branded “insane” on his Facebook account before methodically shooting at people the bar.
One post read: “I hope they call me insane .. wouldn’t that just be a big ball of irony? Yea, I’m insane, but the only thing you people do after these shootings is ‘hopes and prayers’… or ‘keep you in my thoughts’ .. every time .. and wonder why these keep happening….”
Facebook spokeswoman Sarah Pollack said: “Our hearts are with the victims and families affected by this horrendous act. We've removed the shooter's accounts from Facebook and Instagram."
The company also vowed to remove any praise or support for the crime or the shooter.
While Investigators have not commented on whether mental illness played a role in the Mr Long’s attack, a mental health specialist who assessed him in April after officers responded to a call about his agitated behaviour, suspected that he might be suffering from post-traumatic stress disorder (PTSD).
A neighbour had complained of yelling and loud banging noises coming from the home the soldier, who had served in Afghanistan shared with his mother.
Although a mental health crisis team was called to the scene, a specialist concluded there were no grounds to have him involuntarily committed.
Authorities have yet to determine a motive for the massacre but investigators are looking into whether Mr Long believed his former girlfriend was in the bar, a police official told the The Associated Press on condition of anonymity.
Meanwhile, President Donald Trump described the gunman as “a very sick puppy” who had “a lot of problems.”
At the White House briefing, the US leader ignored questions about stricter gun control and instead touted his efforts to fund a program to help veterans suffering from PTSD.