But according to interviews with Rodger's acquaintances, law enforcement officials and mental health professionals, all that was known about the 22-year-old college student was that he was terribly sad.

In that quiet space he planned his attack - lonely, but highly functioning; worrisome, but never explicitly threatening himself or anyone else; bumping into police, but never landing in jail; resistant to medication, but never outright rejecting care; able to articulate his misery, but conniving enough that authorities did not see a need for involuntary admission to hospital.

The writing may have been on the wall, but no one - despite the apparently diligent efforts of his family, therapists and doctors, and law enforcement - knew what it said until it was too late.

"When you looked at him, there was no reason to get concerned. He didn't seem like a threatening or intimidating guy. He was just very quiet," said Chris Pollard, who lived in the same building as Rodger.

Watch: 'Elliot Rodger's Retribution' video surfaces after deadly Southern California shootings

The son of a Hollywood director and a Chinese Malaysian mother, Rodger grew up privileged in Calabasas and Woodland Hills, Los Angeles. He travelled extensively, to Morocco, Singapore and Britain. All along, he harboured a crushing darkness. His family suspected he was somewhere on the autism spectrum, and he had been in therapy since childhood. He was prescribed psychotropic drugs but declined to take them, he wrote.

Belying the aggressive personality seen on his now-notorious videos, he spoke haltingly and did not look people in the eye, said Simon Astaire, a family friend. "He was fundamentally withdrawn," Astaire said.

The family was caring and attentive, said family friend Adam Krentzman. His father Peter Rodger "is the sweetest, nicest, most genuine, caring person, and he did everything he could," Krentzman said.

But after Rodger turned 18, he started rejecting mental health care that his family provided, Krentzman said. "He turned his back on all of it. At some point, your kid becomes an adult." Rodger became increasingly isolated, sometimes by his own design. His parents divorced when he was seven, which he called a "life-changing event".

He remained close to his mother, but had a falling-out with his father that continued for several years. After the divorce, he said, he realised he was uncool and timid, with "a dorky hairstyle", and became self-conscious about his race.

"I was shy and unpopular.... On top of this was the feeling that I was different because I am of mixed race. I am half-white, half-Asian, and this made me different from the normal fully white kids that I was trying to fit in with."

Bitterness rising, Rodger began to resent the carefree students in the Isla Vista community, where more than half of the 23,000 residents are students at the nearby University of California, Santa Barbara.

He viewed himself as a sophisticate and a catch, and reserved much of his venom for attractive women, who he believed spurned him, and men who had more success in dating.

One night last summer, he went to a party and tried to shove women off a ledge where they had been sitting. Several men intervened and pushed him off the ledge instead, and he injured his ankle.

He was treated at a clinic for his injuries, and police showed up to interview him. In theory, this was an opening for formal, official intervention.

But the officers determined Rodger was "not a victim", a Santa Barbara County sheriff's spokeswoman said - that he had instigated the altercation. About the same time, Rodger hatched his plan for what he called "slaughter" and began buying semiautomatic handguns.

More recently, Rodger began to post videos to YouTube. Some were innocuous. Most, though, were brooding and dark.

At one point, he filmed from his glistening BMW as two young people kissed on a park bench in the distance. "I hate the world because no girl would do this with me," he said.

He wrote that his mother bought him the BMW to give him confidence, but it didn't. Seeing any couple set him off. In particular he vented about Indian, Asian and black men dating the blonde women he desired.

Rodger's parents were disturbed by the videos, family friends said. Mental health professionals put them in touch with the Sheriff's Department. And in April, deputies visited Rodger at his apartment, the same one where he would soon stab and kill three men to launch his rampage.

Rodger would later write that he was terrified when deputies knocked on his door - terrified that they would find his weapons, his ammunition and the detailed written plans of his attack.

Rodger wrote that he "tactfully" told the officers there had been a misunderstanding. The deputies left.

He decided to launch his attack that Friday because if he'd waited past the weekend, the semester would have begun to wind down at the nearby university, and many of his "enemies" would be leaving for the summer.

Exactly how the rampage began remains one of the case's key mysteries. How did a physically unimposing 22-year-old manage to stab three men - Cheng Yuan Hong, 20, George Chen, 19, and Weihan Wang, 20 - to death, apparently without attracting anyone's attention?

Friends said that Hong, Chen and Wang were close friends as well as housemates. They loved playing video games and talking about computer engineering and had a shared Chinese heritage.

Hong emigrated to the United States from Taiwan, Wang was from China and Chen was the son of Chinese immigrants.

At 9.17pm, 13 minutes before the first gunshots rang out, his mother, Chin Rodger, was at home in a western suburb of Los Angeles when her phone rang. It was one of her son's therapists. "Have you gotten Elliot's e-mail?" he asked.

Chin Rodger opened the e-mail, according to an account by a family friend. Something had changed. Her dejected son was gone, replaced by a man with a savage view of the world and a terrible plan.

"This is the story of how I, Elliot Rodger, came to be," the e-mail said. "It is a dark story of sadness, anger and hatred."

Then: "I will punish everyone. And it will be beautiful. Finally, at long last, I can show the world my true worth."

Chin Rodgers frantically called her ex-husband, Peter, who was out to dinner. Together, they raced up the 101 Freeway. But it was too late.

The radio started barking while they were still on the road - a vicious rampage in Isla Vista, a young man emptying his guns into crowds of pedestrians.

By the time they reached the police station there, it was over.