The California bar shooter, around the time the gunfire began, started posting to social media, multiple law enforcement officials told NBC4's I-Team.
Ian David Long, 28, made two separate posts amid his shooting rampage at the Borderline Bar & Grill in Thousand Oaks Wednesday night, the officials said.
Several law enforcement sources characterized Long's first message as one that considered his mental state.
"People will debate for years whether I’m sane or insane," sources paraphrased Long's posts.
Long continued, "but I'm sane, life is bulls**t, who cares, and I’m bored."
The law enforcement sources did not say what exactly Long posted online or which social media platforms he used, but he posted text that referenced how people perceived him as being troubled.
Long's use of social media was first reported by TMZ.
A spokesperson for Facebook, which owns Instagram, declined to comment on whether anything had been posted to Facebook. The spokesperson also said no story was posted on Instagram but wouldn't elaborate further, deferring to law enforcement.
Twitter did not return a request for comment.
On Thursday, 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 and will remove any praise or support for the crime or the shooter as soon as we're aware."
#ThousandOaksStrong: Community Mourns Borderline Bar Victims
Investigators are still piecing together a precise timeline on the shooting that left 12 bar employees and patrons dead. But it's believed that Long, a Marine veteran who was dressed in black and armed with a high-powered Glock handgun with an extended magazine, entered the Borderline Bar & Grill about 11:15 p.m.
The officials said he took a position of cover before fatally shooting Sgt. Ron Helus of the Ventura County Sheriff's Office.
Investigators believe Long then shot additional victims inside the bathroom stalls, as well as at least one person who was trying to escape through the bathroom window, the law enforcement officials said.
Long then shot himself, the officials said.
Authorities said Long, a former Marine gunner who served one tour in Afghanistan, had been experiencing emotional issues.
Investigators are still trying to figure out the motive for the attack.
When authorities entered his home in nearby Newbury Park, they found evidence that he punched multiple holes in the walls, law enforcement officials said.
Local cops were called to his home for a domestic disturbance earlier this year, authorities said. Long was found to be "somewhat irate" and "acting a little irrationally" but he wasn't taken into custody.