A six-year-old boy who was shot in the back on an inflatable bounce house is among the dead in America’s 246th mass shooting this year.

Young Stephen Romero had been attending the popular Gilroy Garlic Festival in California with his mother and grandmother when a gunman opened fire, seemingly at random.

A 13-year-old girl and man in his 20s were among three killed while another 15 were wounded before police shot dead the assailant within one minute of the attack.

The shooter has been identified as Santino William Legan, 19, who had days earlier posted Instagram images from the festival and comments supporting a White Supremacist text.

A screenshot of the killer’s Instagram account which was only activated days earlier.

Security is tight at the festival, with people passing through metal detectors and bag searches, but police say Legan cut through the fence before opening fire with an AK47-style rifle, sending attendees running and screaming.

TinMan singer Jack van Breen, who had been performing an encore on stage when mayhem broke out, said he heard someone shout to the shooter: “Why are you doing this?”.

Then came the teenager’s chilling reply: “Because I’m really angry.”

Garlic festival attendees after the shooting at the California food event. Photo: AAP

The Gilroy Garlic Festival draws about 100,000 people over three days to the city that bills itself as the Garlic Capital of the World, about 130km from San Francisco.

Attendees had been sampling garlic-laced dishes of pasta and seafood, and enjoying live music as children played before they heard the “pop, pop, pop” at first mistaken for fireworks.

Witnesses described the “nightmare” unfolding, with one woman telling CNN the shooter stood between her and escape.

“I started running, we hit the ground and I literally laid down on the ground,” Miquita Price told CNN.

“(There) was blood everywhere. I read about this but I never thought I would be in it.”

Stephen’s father Alberto Romero said he was at home when he got a panicked call from the boy’s mother who has since been placed in an induced coma after being shot in the stomach.

“I couldn’t believe what was happening, that what she was saying was a lie, that maybe I was dreaming,” Mr Romero said.

“He was joyful, always wanted to play, always positive.”

Only six years old, Stephen Romera was shot dead while playing on a bouncy castle.

A website that tracks American shootings, Gun Violence Archive, reported it as the 246th mass shooting this year at locations ranging from festivals to schools, places of worship, movie theaters, workplaces and bars.

Victims of gun violence in America totaled 8,467, with the number of children (aged 0 – 11) killed or injured reaching 373, according to their website.

Gilroy Police Chief Scot Smithee described the scene as a “nightmare you hope you never have to live”.

The wounded were taken to multiple hospitals, and their conditions ranged from fair to critical.

At least five were treated and released.

Some witnesses reported a second suspect, Chief Smithee said, but it was unclear whether that person was armed or simply provided assistance.

A manhunt continued into Monday.

Security measures are taken at the road into Christmas Hill Park after the mass shooting. Photo: Getty

President Donald Trump described the gunman as a “wicked murderer” and asked people to pray for the victims during an event at the White House.

“We grieve for their families, and we ask that God will comfort them with his overflowing mercy and grace,” he said.

Video posted to social media showed people running in terror as shots rang out.

Evenny Reyes, 13, told the San Jose Mercury News that at first she thought the gunfire was fireworks but then saw someone with a wounded leg.

“We were just leaving and we saw a guy with a bandana wrapped around his leg because he got shot,” Evenny said.

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

Maximo Rocha said he saw many people on the ground, but could not be sure how many were shot.

He told NBC Bay Area that “quite a few” were injured, “because I helped a few”.

The killer appeared to post a photograph from the festival on his Instagram account shortly before the shooting accompanied by disgruntled captions.

The account only appeared to be a few days old and was deactivated at some point on Monday morning.

“Ayyy garlic festival time,” he wrote in the caption to the picture of people walking through the festival grounds. “Come get wasted on overpriced shit.”

Earlier

Police and ambulances have raced to a shooting at a food festival in California, with four people reported dead and several injured.

Video posted on social media showed people at the event running for cover as shots rang out.

NBC Bay Area reported initially that ambulance crews were told 11 people were “down” after the shooting.

Later, four people were reported to have died, including the suspected gunman. Fifteen more were injured.

Earlier, Santa Clara Valley Medical Centre spokeswoman Joy Alexiou said the hospital had received two victims from the shooting and expects three more.

“It’s just incredibly sad and disheartening that at an event that does so much good the community had to suffer from a tragedy like this,” said police chief Scott Smithee on Sunday in the city of Gilroy, near San Jose.

The gunman appeared to have cut through a fence to get into the annual Garlic Festival. He might have had one accomplice, Mr Smithee added.

Aerial footage showed the festival grounds apparently deserted but with many emergency and police vehicles on surrounding streets and police in bullet-proof gear.

“Be careful and safe!” US President Donald Trump wrote on Twitter.

Footage uploaded to social media appeared to show festival attendees scattering in fear and confusion as loud popping sounds could be heard in the background.

“What’s going on?” a woman can be heard asking on one video. “Who’d shoot up a garlic festival?”

yo somebody was shooting at the gilroy garlic festival. be safe pic.twitter.com/B39ZIYe8wr

— niah ㊝ (@wavyia) July 29, 2019

California Governor Gavin Newsom said his office was monitoring the situation closely.

Evenny Reyes, 13, told the San Jose Mercury News that at first she thought the gunfire was fireworks. But then she saw someone with a wounded leg.

“We were just leaving and we saw a guy with a bandana wrapped around his leg because he got shot,” Reyes told the newspaper.

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

One video posted on Twitter showed a blood-spattered woman sitting in the back of a semi-trailer and telling a man she had been shot in the hand.

Founded in 1979, the Gilroy Garlic Festival features food, drink, live entertainment and cooking competitions.

Founded in 1979, the Gilroy Garlic Festival features food, drink, live entertainment and cooking competitions.

It was being held at the outdoor Christmas Hill Park, where weapons of any kind are prohibited, according to the event’s website.

-with AAP