(CNN) The 19-year-old gunman who used an assault-style rifle to shoot people at the Gilroy Garlic Festival last week had a "target list" made up of religious institutions and political groups of both parties, as well as federal buildings and courthouses, authorities said.

Given the threats to nationwide organizations, the FBI is opening a domestic terrorist investigation into the shooting, FBI special agent in charge of the San Francisco office John Bennett said.

The FBI has not made a conclusion as to his motivation, and the shooter left no manifesto, Bennett said. The gunman was exploring violent, competing ideologies, and "there was nothing that was all one-sided or the other," he said.

The shooter, Santino William Legan, cut through a back fence to get into the food festival and began shooting at random with a weapon he bought in Nevada weeks earlier, authorities said.

Three people -- 6-year-old Stephen Romero, 13-year-old Keyla Salazar and 25-year-old Trevor Irby -- were killed in the shooting , and more than a dozen people were wounded.

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