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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.
A college country music night was under way when the suspect, apparently dressed in black, entered the bar and opened fire.
Reports say the suspect may have used smoke grenades and at least one weapon, described as a semi-automatic handgun by witnesses.
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 from inside, Teylor Whittler, 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.
Ventura Sheriff's Sergeant Ron Helus, who later died in hospital, had tried to enter the active scene with a local highway patrolman when he was shot several times.
The husband and father-of-one had been a policeman for 29 years and was due to retire next year.
"It saddens us all and tears at our emotions. He died a hero. He went in to save lives, to save other people," the sherriff said.
Police say they recovered a handgun inside the bar and found the suspect dead inside.
They believe he may have taken his own life but have not confirmed this.
It is not yet clear who the other 11 victims inside the bar are, but the Wednesday night country music event is popular with local university students.
Footage on local media of the aftermath shows injured young people being carried away from the scene by friends.
Police say at least 10 people are known to have been injured and others are self-reporting with injuries at local hospitals.
The US has suffered from mass shootings for decades, but most of the deadliest have occurred in recent years.
In the last two weeks alone, a man shot dead two people yoga studio in Florida, and another gunman opened fire on a synagogue in Pittsburgh, killing 11.
The deadliest attack in modern US history came last year in Las Vegas when a 62-year-old man opened fire on a crowd of 22,000 country music concertgoers from the 32nd floor of a nearby hotel, killing 58 and wounding hundreds.
The previous most deadly attack was another just a year earlier, when 49 people were killed in Pulse - a gay nightclub - in June 2016. in Orlando.
Some students from Marjory Stoneman Douglas High School in Parkland, Florida - where 17 were killed in February - have taken up a public campaign for gun law reform in recent months but little in the way of change has emerged from legislators.
According to the website Gun Violence Archive, more than 12,000 people have been killed using firearms in the US so far this year, including about 3,000 people under 18.
That number does not include an annual estimate of 22,000 suicides via firearm.
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