At least 12 people have been killed, including a police officer, at a country music bar in Thousand Oaks, California.
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.
Officials say the suspect, who has not been identified, was found dead inside.
They have not yet confirmed any motive behind the attack.
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.
I have been fully briefed on the terrible shooting in California. Law Enforcement and First Responders, together with the FBI, are on scene. 13 people, at this time, have been reported dead. Likewise, the shooter is dead, along with the first police officer to enter the bar....
Witnesses say the suspect was dressed in all black, and forced his way into the bar after first shooting the bouncer.
A college country music night was underway when the suspect apparently threw a smoke grenade into the crowd before opening fire with at least one weapon.
Witnesses told CNN that he used a short-barrelled shotgun, while others told local media that he carried a "smaller-calibre handgun with a light or laser attached to the bottom of it".