At a news conference, FBI special agent in charge John Bennett said about 100 agents from around the country are trying to determine the motive or even an ideology for 19-year-old Santino Legan, who opened fire at the weekend fair southeast of San Francisco.
He was shot to death by three Gilroy police officers.
The investigation includes profilers and agents checking social media.
Bennett says it still appears that Legan picked his targets at random.
Bennett says investigators are making progress clearing the crime scene at the festival, which was held in a huge park. But he says they must account for every bullet fired and other details.
People who took photos or videos of the attack are being asked to contact investigators.