An investigative report on a rampage that killed six University of California, Santa Barbara, students says the killer searched online for ways to silently kill before stabbing to death his first three victims last May.

The Santa Barbara County Sheriff's Office on Thursday released results of its investigation detailing how 22-year-old Elliot Rodger stabbed his first victims, and then drove through the community of Isla Vista, fatally shooting three people and injuring 14 others.

Rodger got into two shootouts with deputies, before fatally shooting himself.

Rodger used two long knives, including a nearly 9-inch boar hunting knife, to kill his two roommates and a visiting friend.

The report says Rodger searched online for the phrase "quiet silent kill with a knife" before likely ambushing each and stabbing them multiple times.