At least 13 people including a police officer were killed after gunman opened fire, at a bar in Thousand Oaks, California, BBC reported.
The shooting began at 23:20 local time on Wednesday about 40 miles (65km) north-west of Los Angeles, police said.
At least 200 people were reportedly inside the Borderline Bar and Grill at the time of the attack.
Officials say the suspect was found dead inside and have not identified him. They do not yet know his motive for the shooting.