LOS ANGELES — Federal authorities have launched a domestic terrorism investigation into the Gilroy Garlic Festival shooting that left three dead after officials discovered the gunman had a list of other potential targets, authorities said at a news conference Tuesday.

FBI Special Agent in Charge John Bennett said the target list included religious institutions, courthouses and political institutions involving both the Republican and Democratic parties. He declined to provide specifics on the targets, but said law enforcement officials were reaching out to notify them.

Santino William Legan, 19, opened fire at the popular food festival the evening of July 28, killing three people and wounding 13 more. Police had previously said his rampage was cut short when three Gilroy police officers engaged a rifle-wielding Legan with handguns and killed him in less than a minute.

Legan killed himself, the county medical examiner concluded Friday, contradicting that earlier police account.

The announcement of a domestic terrorism probe comes as authorities continue to try to determine a motive for the attack and investigate the Legan’s background. Authorities have not determined whether the gunman was a white nationalist, but they have not ruled it out either, Bennett said.

“We have uncovered evidence that the shooter was exploring violent ideologies,” he said.

Authorities are trying to determine who Legan may have been in contact with before the attack, whether anyone helped coordinate it and why he ultimately carried it out, Bennett said.

The FBI has seized from Legan’s residence what a law enforcement source described as extremist materials, but Bennett, special agent in charge of the bureau’s San Francisco office, pushed back Thursday on reports that Legan was motivated by extremist or white supremacist beliefs. Written materials taken from Legan’s residence in Nevada ran the ideological gamut, Bennett said.

FBI profilers are interviewing Legan’s relatives and associates, reviewing his online presence and combing through materials seized from the Nevada residence, which include several hard drives, a computer tower, books and a letter from a relative, according to a receipt of a search warrant released by Nevada authorities.

Bennett said Thursday that there is no indication Legan targeted festival attendees of a particular race. Shortly before the attack, he promoted on Instagram a 19th-century screed often championed by white supremacists, and used a slur to refer to mixed-race people. Legan, whose family lives in Gilroy, identified himself on Instagram as of Italian and Iranian heritage.