Authorities are searching for answers to why a 19-year-old opened fire on a popular food festival about a kilometre from his parents' home in California, killing two children and another man.

Santino William Legan, 19, cut through a fence and appeared to randomly target people with an "assault-type rifle" on Sunday afternoon, the end of the three-day Gilroy Garlic Festival.

Six-year-old Stephen Rome...