The night is popular among the area's local university students. Photo: EPA

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, who had suspected mental health issues.

Officials believe the US Marine Corps veteran and local resident took his own life inside the bar.

Police say the suspect, Ian David Long, was dressed in black, and forced his way into the bar after shooting the bouncer.

A college country music night was under way when the suspect apparently threw a smoke grenade before opening fire, witnesses say.

Police say he used a .45 calibre Glock semi-automatic handgun, which had an extended magazine that allowed it to carry more than its typical 11-bullet capacity.

One witness, Teylor Whittler, who was celebrating her 21st 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 escaped the bar by using chairs to break windows, while others reportedly sheltered inside the venue's toilets.

What have police said?

Ventura County Sheriff Geoff Dean described the scene inside the bar as "horrific" and said there was "blood everywhere".

He said first responders arrived less than 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 a professional firearms instructor.

He 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 sheriff said.

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.

According to the bar's website, Wednesday nights are college-themed and are open to students as young as 18.

The bar is located close to California Lutheran University, Pepperdine University and Moorpark College and is popular among students.