Parents heard rampage news on radio
Hunger Games assistant director Peter Rodger and his ex-wife Chin later learned their British-born son had killed six people and wounded 13 in the college town of Isla Vista before, authorities say, taking his own life.
The Los Angeles Times said Ms Rodger received a call from her son's therapist shortly before Friday's shootings about a ranting email he had sent. She then found his video vowing to kill people.
Family friend Simon Astaire told the newspaper that Ms Rodger alerted authorities and set off in a car with her ex-husband. But by the time they arrived, police said their son had gone on a rampage.
Sheriff's deputies had visited Rodger, 22, last month to check on his mental health but had not seen online videos in which he threatened suicide and violence, even though those recordings were what prompted his parents to call authorities.
By the time police did see the videos, it was too late - the well-mannered, if shy, young man that deputies concluded posed no risk had gone on a deadly rampage, stabbing and shooting dead six people before taking his own life.
Santa Barbara County Sheriff's Office was "not aware of any videos until after the shooting rampage occurred", spokeswoman Kelly Hoover said.
Sheriff Bill Brown has defended the officers' actions, but the case highlights the challenges police face in assessing the mental health of adults, particularly those with no history of violent breakdowns, institutionalisations or serious crimes.
"Obviously, looking back on this, it's a very tragic situation and we certainly wish that we could turn the clock back and maybe change some things," Sheriff Brown told CBS' Face The Nation.
"At the time deputies interacted with him, he was able to convince them that he was OK," he said.
It's not clear why the sheriffs did not become aware of the videos. Lawyer Alan Shifman said the Rodger family had called police after being alarmed by YouTube videos "regarding suicide and the killing of people" that their son had been posting.
Doris Fuller, executive director of the Virginia-based Treatment Advocacy Centre, said California law has provisions that permitted emergency psychiatric evaluations of individuals who posed a serious threat, but that was never triggered.
Rodger's family has said Elliot was under the care of therapists.
"Once again, we are grieving over deaths and devastation caused by a young man who was sending up red flags for danger that failed to produce intervention in time to avert tragedy," Ms Fuller said.
"In this case, the red flags were so big the killer's parents had called police...and yet the system failed."
Rodger, writing in a manifesto, said he was relieved his apartment was not searched because deputies would have uncovered the cache of weapons he used in the beach town rampage.
He posted at least 22 YouTube videos and wrote in his manifesto that he uploaded most of his videos in the week leading up to April 26, when he originally planned to carry out his attacks. He postponed his plan after catching a cold.
Because many of the videos were removed from YouTube then re-added in the week leading up to the killings, it is unclear which of the videos alarmed his family, or whether others were reported that were not uploaded again.
He voices his contempt for everyone from his room-mates to the human race, reserving special hate for two groups: the women he says kept him a virgin for all of his 22 years and the men they chose instead.
At least two other people who saw Rodger's videos before Friday compared him to a serial killer, through a message board on a bodybuilding website and the social network Reddit.
The rampage played out largely as he sketched it in public postings, including a YouTube video where he sits in the BMW in sunset light and appears to be acting out scripted lines and planned laughs.
"I'll take great pleasure in slaughtering all of you," Rodger says in the video posted on Friday and taken down by YouTube on Saturday with a message saying it violated the site's terms of service.
Sheriff Brown told CNN investigators are close to having a "pretty clear picture of what happened".
The first three killed were male stabbing victims in Rodger's own apartment Brown said Saturday. Then, at about 9.30pm, the shooting rampage began.
The Santa Barbara sheriff's office said the victims were University of California Santa Barbara students Cheng Yuan Hong, 20, 19-year-old George Chen and Weihan Wang, 20.
Mr Hong and Mr Chen are listed on the lease as Rodger's room-mates and investigators are trying to determine whether Mr Wang was also a room-mate or was visiting on the night of the killings.
The shooting victims were Katherine Cooper, 22, Christopher Michael-Martinez, 20 and Veronika Weiss, 19.