British-born gunman Elliot Rodger stabbed and killed his college room-mates before he embarked on a shooting rampage, police said.
Rodger, 22, carried out a deadly gun and knife rampage near Santa Barbara, California after emailing a chilling manifesto outlining his intentions to his parents and therapist.
The first three victims killed in the spree on Friday night were stabbed in Rodger's own apartment.
They were University of California Santa Barbara students Cheng Yuan Hong, 20, 19-year-old George Chen and Weihan Wang, 20, the Santa Barbara sheriff's department said.
A statement from the department said: "They were found deceased with multiple stab wounds in Rodger's apartment located in the 6500 block of Seville Road in Isla Vista. Hong and Chen are listed on the lease for the apartment as tenants, along with Rodger.
"Sheriff's investigators are in the process of determining whether Wang was also a room-mate or was visiting the residence."
The statement added: "Sheriff's investigators believe Rodger killed these victims prior to going on a shooting rampage in Isla Vista around 9.27pm on May 23. In all, six innocent people were killed and 13 others were wounded. Rodger died in the incident, possibly from a self-inflicted gunshot wound."
An additional statement from the sheriff's department confirmed Katherine Cooper, 22, and Veronika Weiss, 19, were shot outside the nearby Alpha Phi sorority house.
It said: "Several women in the sorority reported hearing loud and aggressive knocking on the front door for several minutes. Fortunately, no one opened the door and the suspect left.
"Shortly afterwards, witnesses reported seeing three women shot by the suspect across the street. Two of the women, Katherine Cooper and Veronika Weiss, died from their injuries. A third victim received several gunshot wounds and was transported to the hospital."
The sixth victim, Christopher Michael-Martinez, 20, was shot at a nearby delicatessen.
Rodger's parents desperately searched for him as he carried out the attacks after they received his chilling manifesto, it has emerged.
They heard the news of his bloody rampage on the radio as they were racing to Santa Barbara after his mother saw his online threats.
The manifesto details his rejection by women and his fury at men who find it easy to attract the opposite sex. It also chronicles his distress about his height, appearance and his parents' divorce.
Rodger's mother, Li Chin, saw the email at 9.17pm local time and immediately went to her son's YouTube page, where she saw a video entitled Retribution that he posted on the day of the killings, family friend Simon Astaire told CNN.
In the video, Rodger spoke of his plan to "slaughter" women at a sorority house at the University of California at Santa Barbara.
Mr Astaire told the news network Rodger's father - Hollywood director Peter Rodger, who worked on The Hunger Games - and mother raced to Santa Barbara to find their son after they received the document.
On the way to the seaside town they heard there was a shooting and later that night, they found out their son was behind the violence, the family friend said.
The shootings began at 9.30pm local time on Friday, minutes after Rodger's mother read the manifesto.
Christopher Michael-Martinez's father Richard spoke with his son 45 minutes before he died. He told reporters: "Our family has a message for every parent out there - you don't think it will happen to your child until it does.
"Chris was a really great kid, ask anyone who knew him. His death has left our family lost and broken.
"Why did Chris die? Chris died because of craven, irresponsible politicians and the NRA (National Rifle Association). They talk about gun rights, what about Chris's right to live?
"When will this insanity stop? When will enough people say, 'Stop this madness, we don't have to live like this'. Too many have died. We should say to ourselves, 'Not one more'."
Rodger, who had Asperger's syndrome, was found with three 9mm semi-automatic guns in his car and more than 400 rounds of unused ammunition.
Santa Barbara County Sheriff Bill Brown said he had "no doubt there would have been further loss of life" if police had not intervened.
The Rodger family's lawyer, Alan Shifman, revealed that they called police several weeks ago after being alarmed that the future killer had uploaded several YouTube videos "regarding suicide and the killing of people".
Police interviewed him at his home but found him to be a "perfectly polite, kind and wonderful human", Mr Shifman said.
Rodger had posted videos and written blogs saying he would carry out the killings because he was a virgin and had never kissed a girl.
The "Retribution" YouTube video shows Rodger sat in his car, looking directly at the camera and declaring "the day of retribution" was coming. He describes plans to shoot women and promises retribution for his "loneliness and frustration" at never having had a girlfriend.
The student went on: "For the last eight years of my life, ever since I hit puberty, I've been forced to endure an existence of loneliness, rejection and unfulfilled desires all because girls have never been attracted to me."
Rodger said: "All you popular kids, you've never accepted me and now you'll pay for it."
He went on: "I'll take great pleasure in slaughtering all of you. You will finally see that I am, in truth, the superior one, the true alpha male."
The 141-page "manifesto" outlined his plans and talked of how he narrowly evaded being found out when police knocked on his door.
"I had the striking and devastating fear that someone had somehow discovered what I was planning to do, and reported me for it," Rodger said in the document.
"If that was the case, the police would have searched my room, found all of my guns and weapons, along with my writings about what I plan to do with them. I would have been thrown in jail, denied of the chance to exact revenge on my enemies.
"I can't imagine a hell darker than that. Thankfully, that wasn't the case, but it was so close," he wrote.
"The police interrogated me outside for a few minutes, asking me if I had suicidal thoughts. I tactfully told them that it was all a misunderstanding, and they finally left," he added.
"For a few horrible seconds I thought it was all over. When they left, the biggest wave of relief swept over me. It was so scary,"
Mr Brown revealed that the three guns found in Rodger's car - two Sig Sauer P226 handguns and a Glock 34 Long Slide - were legally bought from licensed firearms dealers and were registered in his name.
Mr Brown added officers had come into contact with Rodger three times over the last year, including in January when he performed a citizen's arrest on his room-mate for stealing a candle, and last month when Rodger's family asked police to check on him after they were concerned for his welfare.
He said officers found him "polite and courteous" and saw no reason to detain him on mental health grounds. He said he refused to "second guess" his officers' decision.
Mr Shifman said Rodger had received help from "multiple therapists", and added his social worker was sufficiently concerned about him to call the police last week.
He said it was a tragedy of "immense consequences" and that the Rodger family was co-operating fully with police.
Mr Shifman said: "My client's mission in life will be to try to prevent any such tragedies from ever happening again. This country, this world, needs to address mental illness and the ramifications from not recognising these illnesses."
He added that the family was "staunchly against guns" and supported gun-control laws. "They are extremely, extremely upset that anybody was hurt under these circumstances," he said.