A law enforcement official said the gunman was identified as Santino William Legan and believed he was 19. The official wasn't authorized to discuss the matter publicly and spoke to The Associated Press on condition of anonymity Monday.
The shooter appeared to randomly target people when fired with a rifle Sunday afternoon, the end of the three-day Gilroy Garlic Festival that attracts more than 100,000 people to the city known as the "Garlic Capital of the World," Gilroy Police Chief Scot Smithee said.
The shooter sneaked in through a fence that borders a parking lot next to a creek. Police responded within a minute and killed the suspect, Smithee said.
Some witnesses reported a second suspect, Smithee said, but it was unclear whether that person was armed or simply provided assistance. A manhunt continued late into the night.
Six-year-old Stephen Romero was among those killed, his father said.
"My son had his whole life to live and he was only 6," Alberto Romero told San Francisco Bay Area news station KNTV. "That's all I can say."
The boy's grandmother, Maribel Romero, told Los Angeles station KABC-TV that she searched several hospitals before learning he had died. She said he was "always kind, happy and, you know, playful."
The wounded were taken to multiple hospitals, and their conditions ranged from fair to critical, with some undergoing surgery. At least five were treated and released.
The Gilroy Garlic Festival features food, cooking contests and music. It's a decades-old staple in the agricultural city of 50,000 about 80 miles (176 kilometers) southeast of San Francisco. Festivalgoers pass through metal detectors and their bags are searched at the event.
The band TinMan was starting an encore Sunday when the shooting started. Singer Jack van Breen said he saw a man wearing a green shirt and grayish handkerchief around his neck fire into the food area with what looked like an assault rifle. He and other members of the band dove under the stage.
Van Breen, from nearby Santa Clara, said he heard someone shout: "Why are you doing this?" The reply: "Because I'm really angry."
The audience began screaming and running, and the five members of the band and others dove under the stage.
Bandmate Vlad Malinovsky said he heard a lot of shots and then it stopped. Later, law enforcement told those hiding to come out with their hands up.
Taylor Jackson was working at a booth drawing caricatures when she heard gunfire, saw people running and "ran for the hills." She said her boss ran in the opposite direction. Several hours later, Jackson was at a reunification center trying to get information on her whereabouts.
Donna Carlson of Reno, Nevada, was helping a friend at a jewelry booth when "all of a sudden it was pop, pop, pop. And I said, 'I sure hope that's fireworks.'" She got on her hands and knees and hid behind a table until police told her it was safe to leave.
In a tweet, California Gov. Gavin Newsom called the bloodshed "nothing short of horrific" and expressed appreciation for the police response. President Donald Trump tweeted before authorities confirmed the gunman was dead and urged people to "be careful and safe!"
Video posted to social media showed people running in terror as shots rang out.
Evenny Reyes of Gilroy, 13, told the Mercury News in San Jose that she spent the day at the festival with her friends and relatives.
"We were just leaving and we saw a guy with a bandanna wrapped around his leg because he got shot. And there were people on the ground, crying," Reyes said. "There was a little kid hurt on the ground. People were throwing tables and cutting fences to get out."
Reyes said that she didn't run at first because the gunshots sounded like fireworks.
"It started going for five minutes, maybe three. It was like the movies — everyone was crying, people were screaming," she said.