The 19-year-old gunman who opened fire at a Northern California food festival was "kind of a loner" and much of his life was shrouded in mystery, the FBI said Tuesday as investigators searched for a motive.

Police believe Santino William Legan fired randomly Sunday, killing three people, after cutting through a fence to get into the Gilroy Garlic Festival. Officers patrolling the popular event responded within a minute and killed him. Legan attended high school in Gilroy in his senior year and was recently living in Nevada, where he purchased two guns — the AK-47-style semi-automatic rifle he used in the attack and a shotgun that was found in his car near the festival, authorities said. A bag of ammunition was found in a creek near the fence, police said.

"We understand him to be kind of a loner," said Craig Fair, deputy special agent in charge of the FBI's San Francisco Division.

Legan was living in an apartment in Walker Lake, a remote northern Nevada community, and had not had any run-ins with the law, officials said. "He appears to have moved into Mineral County this spring and maintained a low profile," Sheriff Randy Adams said in a statement. Officials searched the apartment, seizing empty shotgun and rifle boxes, a gas mask, empty ammunition boxes, electronic devices, pamphlets on guns and a sack full of ammunition casings, prosecutors said. Authorities don't believe he was targeting anyone based on any specific characteristics like race, but they're still trying to determine his ideology, Fair said.

Gilroy Police Chief Scot Smithee told reporters that authorities believe Legan acted alone but are still investigating. Investigators tracked Legan's movements around town before the shooting, and video shows him visiting stores alone, Smithee said.