At least 12 people have been killed, including a police officer, at a country music bar in Thousand Oaks, California, BBC reports.
The shooting began at 23:20 local time on Wednesday about 40 miles (65km) north-west of Los Angeles, police say.
At least 200 people were reportedly inside the Borderline Bar and Grill, which was hosting a university student night, at the time of the attack.
Police have named the suspect as 28-year-old Ian David Long.
Officials believe the US Marine Corps veteran and local resident took his own life inside the bar.
President Trump tweeted on Thursday morning to say he been briefed on the shooting, describing it as "terrible", and praising the bravery of the police officers involved.