A GUNMAN who killed three people at a food festival in California on Sunday urged his Instagram followers to read a 19th century book popular with white supremacists on extremist websites.

Santino William Legan posted the caption about the book “Might is Right,” which claims race determines behavior. It appeared with a photo of Smokey the Bear in front of a “fire danger” sign and also complained about overcrowding towns and paving open space to make room for “hordes” of Latinos and Silicon Valley whites.

In his last Instagram post Sunday, Legan sent a photo from the Gilroy Garlic Festival. Minutes later, he shot into the crowd with an AK-47 style weapon, killing a six-year-old boy, a 13-year-old girl and a man in his mid-20s.

Under it, he wrote: “Ayyy garlic festival time” and “Come get wasted on overpriced” items. Legan’s since-deleted Instagram account says he is Italian and Iranian.

‘Let’s get out of here’

The postings are among the first details that have emerged about Legan since authorities say he appeared to fire at random, sending people running and diving under tables.

Video footage showed people fleeing in terror as rifle rounds rang out at one of the largest food events in the country, which attracts around 100,000 each year to the city of Gilroy, 130 kilometres southeast of San Francisco.

Police patrolling the event responded within a minute and killed Legan as he turned the weapon on them.

The gunman legally purchased the semi-automatic assault rifle this month in Nevada, where his last address is listed. He would have been barred from buying it in California, which restricts firearms purchases to people over 21. In Nevada, the age limit is 18.

The Mercury News reported that the shooting took place as the festival was winding down.

Stagehand Shawn Viaggi hit the ground after hearing “loud pops,” it said.

“I called out, ‘It’s a real gun, let’s get out of here,’ and we hid under the stage,” Viaggi said, according to the newspaper.

The Gilroy Garlic Festival features live entertainment including bands and cooking competitions as well as food and drink, according to organisers.

The family event is a major focal point for the small city of around 50,000.

Thirteen-year-old Evenny Reyes told the Mercury News that “we were just leaving and we saw a guy with a bandana wrapped around his leg because he got shot. And there were people on the ground, crying.”

“There was a little kid hurt on the ground. People were throwing tables and cutting fences to get out,” said Reyes.

‘Tough road ahead’

Mass shootings are a frequent occurrence in the US, but despite the scale of the gun violence problem, efforts to address it legislatively have largely stalled at the federal level.

Hundreds of people came out last night for a candlight vigil in front of City Hall in honor of those killed and injured.

“We cannot let the bastard that did this tear us down,” Mayor Roland Velasco declared to cheers.

President Donald Trump paid tribute to the victims at an event at the White House.

“While families were spending time at a local festival, a wicked murderer opened fire and killed three innocent citizens, including a young child,” Trump said.

We grieve for their families and ask that god comfort them.

House speaker Nancy Pelosi, whose district covers most of San Francisco, spoke of the survivors’ “tough road ahead” and called for action to end gun violence.

“Every day the Senate refuses to act is a stain on the conscience of our nation,” she said.

Shooter Legan grew up less than a mile from the park where the city known as the “Garlic Capital of the World”.

Authorities are looking for clues, including on social media, as to what caused the son of a prominent local family to go on a rampage. His father was a competitive runner and coach, a brother was an accomplished young boxer and his grandfather had been a supervisor in Santa Clara County.

Police said they don’t know if people were targeted, but at this point, it appears he shot indiscriminately. Twelve people were also injured during the shooting. 

Police searched Legan’s vehicle and the two-story Legan family home, leaving with paper bags. Authorities also searched an apartment they believed Legan used this month in remote northern Nevada. Officials didn’t say what they found.