No fewer than 12 people, including a policeman, have been killed at a country music bar in Thousand Oaks, California.
The suspect, the Police, disclosed is a 28-year-old Ian David Long.
Officials believe the US Marine Corps veteran and local resident took his own life inside the bar.
The shooting, according to the British Broadcasting Corporation, BBC, began at 23:20 local time on Wednesday about 40 miles (65km) north-west of Los Angeles, police say.
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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.
President Trump tweeted on Thursday morning to say he had been briefed on the shooting, describing it as “terrible.”
He praised the bravery of the police officers involved.
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 under way 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.”
An injured man told local television channel KTLA: “We just dropped to the ground, we heard a lot of screaming. My friend is the DJ so she cut off the music, we just heard a lot of mayhem.”
Another witness, Teylor Whittler, who was celebrating her 21th birthday said: “I was on the dance floor and I heard the gunshots, so I looked back and then all of a sudden everyone screamed: Get Down!
“It was a huge panic, everyone got up, I was trampled, I was kind of left on the floor until some guy came behind me and grabbed me and dragged me out.”
People reportedly escaped the bar by using chairs to break windows, while others reportedly sheltered inside the venue’s toilets.
Ventura County Sherriff Geoff Dean described the scene inside the bar as “horrific” and said there was “blood everywhere”.
He said first responders arrived just three minutes after the first emergency calls from the venue were made.