The shooting took place at the Gable House Bowl in Torrance, 20 miles from downtown Los Angeles.
No other mass shooting that is among the 10 deadliest in America's modern history happened in a small town
A police official identified the gunman as Kevin Downing, a man in his late 60s from Fort Lee, New Jersey, according to media reports.
The White House announced in a statement that the FBI would be brought in to investigate the case.
A North Carolina man has been charged with three counts of first-degree murder.
A 12-year-old boy carrying a realistic-looking pellet gun has been shot by police