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 6,500 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 called 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."