(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.
The gunman was shot multiple times by three responding police officers, and he then killed himself with a self-inflicted gunshot wound to the head, Gilroy Police Chief Scot Smithee said last week.
In a search of his digital media, the FBI found a list of nationwide organizations that may have been potential targets. Those include religious, government and political organizations, including federal buildings, courthouses, and the food festival.
The gunman was wearing body armor when he fired 39 total rounds into the crowd, Smithee said. He had a 75-round drum magazine that fit into his weapon, and there were 71 rounds left in the drum when he died. In addition, he had two 40-round magazines on his body, two 40-round magazines on the ground, and one 40-round magazine that was on or near him, Smithee said.
A Remington shotgun was found in his vehicle, and his nearby backpack had two loose rounds, a rifle scope, flashlight, shovel and buckshot rounds for the shotgun.
An Instagram account bearing the shooter's name, created days before the shooting, posted two messages shortly before the attacks. One post was a photo of people walking around the Garlic Festival, and another post made a reference to a white supremacist text.
Law enforcement sources previously told CNN of finding extremist material in the shooter's Nevada apartment.
Family releases statement
Also on Tuesday, Legan's family released a statement through their attorney saying they were "deeply shocked and horrified" by his actions.
"We have never and would never condone the hateful thoughts and ideologies that led to this event, and it is impossible to reconcile this with the son we thought we knew," the family said. "Our son is gone, and we will forever have unanswered questions as to how or why any of this has happened."
They apologized to the families of the victims and said that every member of the family had cooperated with the investigation.
"To the City of Gilroy and to everyone affected, we are tremendously sorry. No words can begin to express this," the family said.
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