The shooter did it in minutes on Sunday, the police said, even with tight security, in a state where such weapons are heavily restricted and at a time when many Americans are on guard for the next mass shooting.
The suspect, Santino William Legan, wasn't on law enforcement's radar before the shooting, a law-enforcement official said, according to Efe news.
He purchased a WASR-10 AK-47-style rifle in Nevada where such guns are legal and can be sold to anyone 18 and over, law-enforcement officials said. And he avoided the festival's public entrance where security was checking bags by cutting through a fence that surrounded the grounds, the police said.
"It has always been a safe place for us," said Cheryl Low, 56, who was working at a vendor's booth at the festival. "The security was fine out there. When people want to do this they know how, and what they are doing, and they are going to do it regardless."
The attack could have been far more devastating if not for three of the many officers posted at the festival who were on the scene in less than a minute after the first reports of gunfire at 5.41 p.m.
The officers were able to kill him "despite the fact they were outgunned with handguns against a rifle", said Gilroy police chief Scott Smithee on Monday.
Investigators were yet to uncover a motive. On Monday, police cars were lined in front of the suspected shooter's family home in the suburban neighbourhood here.
A post made on an Instagram account bearing Legan's name just before the attack said: "Ayyy garlic festival time Come get wasted on overpriced shit." Another post ranted about paving over open space to make room for "mestizos and Silicon Valley white tws".
On Monday, Big Mike's Guns and Ammo in Fallon, Nevada, where he bought the gun, said on Facebook that Legan seemed normal when he picked up the rifle.
Legan purchased his AK-47-style rifle there on July 9, officials said. It appears that he was a resident of Nevada when he made the purchase.
Such guns can be modified to comply with California's tight restrictions on what the state classifies as assault weapons, but the one the shooter used wasn't, said officials.
It had a detachable magazine and investigators found multiple 30-round magazines at the scene of the shooting. In California, the legal age for purchasing a rifle is 21.
Smithee said investigators haven't found confirmation that there was a second suspect involved in the shooting. Dozens of police officers were searching a wooded area outside the fairgrounds for a second suspect who may have helped, the police said.
( With inputs from IANS )