The FBI has opened a domestic terrorism investigation into a mass shooting at a California festival last month after finding evidence of ‘violent ideologies’ on the gunman’s digital devices, officials said Tuesday.

Santino William Legan killed three people including two children at the Gilroy Garlic Festival in northern California on July 28 before being confronted by police and shooting himself.

The 19-year-old had compiled a list of potential targets including religious institutions, federal buildings, courthouses and both the Democratic and Republican political parties, as well as the festival, said special agent John Bennett

‘Due to the discovery of the target list as well as other information we encountered in this investigation, the FBI has opened a full domestic terrorism investigation into this mass shooting,’ he told a news conference in Gilroy, 80 miles (130 kilometers) southeast of San Francisco.

Legan, who used an AK47-type assault rifle, was wearing a bullet-proof vest during the attack, said Gilroy police chief Scot Smithee.

‘The suspect was hit by the police officers’ gunfire multiple times,’ he said.

‘None of the people who died in this incident were struck by friendly fire. They were all killed by the suspect,’ he added.

A coroner’s report declared Legan’s cause of death was suicide following a gunshot wound to the head.

The FBI requires evidence that an individual committed their act of violence due to ideological motivation to open a domestic terrorism investigation, Bennett said.

The identities of the specific targets will not be made public while the investigation is ongoing but those on the list were being notified, he added.

‘We have not made a final investigative conclusion into the motive of the shooter,’ said Bennett, describing the gunman’s ideology as ‘fractured’ and featuring ‘varying competing violent ideologies.’ Legan’s social media, including an Instagram account created days before the shooting, appeared to reference a white supremacist book.

The killings marked the latest chapter of America’s epidemic of gun violence, which has left nearly 300 people dead in mass shootings so far this year, according to Mass Shooting Tracker.

A week later, the US was rocked by two further mass shootings in Texas and Ohio that killed 32 people.

The El Paso, Texas suspected gunman, who is white and from the Dallas area, reportedly posted an online manifesto railing against a ‘Hispanic invasion of Texas.’

Critics have pointed out that the language echoed much of President Donald Trump’s rhetoric on Twitter and at rallies, where he has frequently characterised Hispanic migrants as part of an ‘invasion.’