Federal authorities have launched a domestic terrorism investigation into the Gilroy Garlic Festival shooting in California that left three dead, authorities said at a news conference on Tuesday.

The move came as officials are still trying to determine a motive for the attack and are digging deeper into the background of the gunman, who shot and killed himself according to the county medical examiner, reports the Los Angeles Times.

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The FBI has seized from the gunman’s residence what a law enforcement source described as extremist materials. But John F. Bennett, special agent in charge of the bureau’s San Francisco office, says authorities have not determined whether he was a white nationalist, but they have not ruled it out either. Written materials taken from the gunman's residence in Nevada ran the ideological gamut, Bennett said.

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Bennett said Thursday that there is no indication the gunman 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. The gunman, whose family lives in Gilroy, identified himself on Instagram as of Italian and Iranian heritage.