FBI Special Agent in Charge of the San Francisco division, John Bennett, revealed at an August 6 press conference that Legan, 19, of Gilroy, had been studying "violent ideologies" before the local massacre.
"I know nothing that we say or do can ever bring back your loved ones but. everything is being done with you in our minds", Bennett said.
Because of the discovery of the target list and other information, the FBI had opened a "full domestic terrorism investigation", he said.
"Much of our work remains ongoing", Bennett said. "Hence the need for thorough, methodical investigation".
He revealed investigators discovered a "target list" compiled by the gunman, that included nationwide religious institutions, federal buildings, courthouses and locations identified with both major political parties - but didn't include the names of people. Authorities said that while they have not determined a motive for last Sunday's shooting, Legan appeared to be interested in collecting conflicting ideologies, with no specific motive for the shooting.
Three people were killed and 13 were injured in the attack on the Garlic Festival, an annual celebration that draws as many as 100,000 visitors that has become synonymous with the city of Gilroy.
Authorities named the three victims killed in the attack as six-year-old Stephen Romero, 13-year-old Keyla Allison Salazar and Trevor Deon Irby, 25.
Members of Legan's family could not immediately be reached for comment.
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Also on Tuesday, Legan's family released a statement through their attorney saying they were "deeply shocked and horrified" by his actions. "No words can begin to express this", the family said. 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 shot himself at some point during that confrontation. He was wearing a bullet-resistant vest and carrying several high-capacity magazines, with more ammunition on the ground and in a bag in a nearby creek.
Smithee said that he finds it sad that people commit such violent crimes targeting strangers. He added that none of the deceased victims was hit by police gunfire.
According to United States media, on the day of the attack Legan had urged his Instagram followers to read a 19th Century book popular with white supremacists and far-right groups. He was also carrying two additional 40-round magazines on his person.
The FBI is opening a domestic terrorism investigation into the shooting that killed three people, including two children, at a popular California food festival, a law enforcement official said Tuesday.
Federal investigators have fewer tools and legal powers at their disposal in domestic terrorism cases than they do if they are up against someone tied to an worldwide organization such as Isis or al-Qaida.
FBI Agents Association president Brian O'Hare called on Congress to codify domestic terrorism as a federal crime. "Acts of violence meant to intimidate civilian populations or to influence or affect government policy should be prosecuted as domestic terrorism regardless of the ideology behind them". The association represents 14,000 active and former Federal Bureau of Investigation special agents.
Bennett said that "everything" is being done with the victims in mind.