LOS ANGELES (AFP) - A disturbed student who left six people dead after a rampage in California spent more than a year planning his killing spree but authorities were largely powerless to prevent the tragedy, an official report found.

Elliot Rodger, 22, son of Hollywood director Peter Rodger, took his own life after killing six people in the town of Isla Vista, outside the University of California at Santa Barbara, in May last year.

A report on Thursday into the massacre by the Santa Barbara County Sheriff's Office noted that several law enforcement officers had visited the student on April 30 after receiving a tip from Rodger's worried mother.

"There was nothing during the contact with the suspect that gave deputies reason to believe he was a danger to himself or others," the report found.

"Such risk factors are required in order to place someone on an involuntary mental health hold, or to legally search their residence."

At the time, Rodger, who had been treated for mental illness since childhood, had already stockpiled an arsenal of firearms and ammunition at his apartment.

He later revealed that sheriff's deputies would have found the weapons had they conducted a search.

Rodger's parents were "shocked" to discover their son had acquired the weapons used in the attack "since he had never shown any previous interest in guns," the report noted.

It said Rodger had spent nearly US$2,500 (S$3,397) on guns and ammunition from December 2012.

- Tragedy prompted law change -

The failure of deputies to search Rodger's residence ultimately prompted a change in the California law.

Under a law passed in August, California law enforcement officers are now encouraged to search the state's gun registry database before carrying out a welfare check on an individual.

The shooting report applauded the changes to California law but warned that several "unmet needs" remained.

"These include greater recognition and early intervention in cases of mental illness, improved community-based treatment and providing more forensic mental health facilities and programs as alternatives to jail," the report concluded.

Rodger stabbed to death three men in his apartment before going on a shooting spree nearby and killing two young women and another young man.

One of Rodger's victims, 19-year-old George Chen, had been stabbed 94 times, the report revealed.

Another 13 people were wounded during his attack, which Rodger had talked about in a chilling 140-page manifesto titled "My Twisted World."

Thursday's report revealed that an examination of Rodger's laptop following the attack had shown that the student had searched for information on mass knife attacks, including one in the Chinese city of Kunming last year which left 29 people dead.

He had also searched for information on Adolf Hitler.

Police also recovered journals written between April 2010 and May 2014 which chronicled his frustration at remaining a virgin and his fantasies about acquiring vast wealth through winning the lottery.

"I will be able to fully indulge in everything I'm passionate about: exotic cars, luxurious mansions, expensive clothes, and of course, sex with beautiful girls," Rodger wrote in one entry.

"I will drive a Maserati and a Lamborghini and when girls see me in those cars they will all want to date me .. I just know it!"