Three people were killed and at least 15 others injured in a shooting at a major food festival in California on Sunday, police said.

Officers confronted and shot dead the suspect "in less than a minute," said Scot Smithee, police chief of the city of Gilroy, 30 miles (48 kilometers) southeast of San Jose.

Police said it was "still an active crime scene" at the site of the three-day Gilroy Garlic Festival, one of the largest food festivals in the country.

A search for a possible second suspect was ongoing, Smithee said.

"We believe based on witness statements that there was a second individual involved in some way, we just don't know in what way," he told journalists.

Footage showed people fleeing in terror as shots rang out.

A witness named Julissa Contreras told NBC a white man in his 30s armed with a ­rifle opened fire indiscriminately.

"I could see him shooting in just every direction. He wasn't aiming at anyone specifically. It was just left to right, right to left," Contreras said, according to NBC.

"He definitely was prepared for what he was doing." 

Smithee said the suspect appeared to have entered the 50-acre festival site via a creek, using cutting equipment to breach the event's perimeter.

Metal detectors and bag searches were in place as part of "very tight security" at the festival, he said.

No motive was known but the suspect's shooting appeared "somewhat random," Smithee added. 

"The suspect used some sort of a rifle," he said.

US President Donald Trump earlier tweeted that law enforcement officers were at "the scene of shootings" in Gilroy, urging members of the public to "Be careful and safe!"

The Gilroy Garlic Festival features live entertainment including bands and cooking competitions as well as food and drink, and draws nearly 100,000 people over three days each year, according to organizers.

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

California governor Gavin Newsom said the incident was "nothing short of horrific."

"Tonight, [California] stands with the Gilroy community. My office is monitoring the situation closely," he tweeted. 

"Grateful for the law enforcement's efforts and their continued work as this situation develops."

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

Stage hand 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.

Thirteen-year-old Evenny Reyes told the paper 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.

"The hearts of Gilroy PD and entire community go out to the victims of today's shooting at the Garlic Festival," the local police department ­tweeted.