The gunman who was shot and killed by police after the attack has been identified as Santino William Legan, 19, CBS has reported.
Earlier, CNN said that his motive in opening fire at a popular family event remained unclear.
According to Gilroy police, the crime scene is still active. Witnesses reported seeing another man who could have been involved in the shooting and a manhunt is underway for the possible accomplice, CNN reports, quoting Gilroy Police Chief Scot Smithee.
The attack has claimed the lives of three people, including a 6-year-old boy. Twelve more people were injured.
Gilroy, widely known for its garlic production, has hosted an annual food festival at Christmas Hill Park since 1979.