At least 12 people have been killed, including a police officer, at a bar in Thousand Oaks, California, the BBC reports.
Police say the shooting began at 23:20 local time on Wednesday at the Borderline Bar and Grill about 65km north-west of Los Angeles.
At least another 10 were injured in the attack.
Officials say the suspect was found dead inside and have not yet identified him. They do not yet know his motive for the shooting.
At least 200 people were reportedly inside the bar attending a college country music night at the time.
"When deputies arrived they heard shots being fired, it was an active shooter situation," Ventura County Sheriff's office spokesman Eric Buschow said earlier.