Updated | 12 killed in California nightclub mass shooting, gunman identified

The gunman who killed 12 people have been killed and injured several after opening fire at a nightclub in Thousand Oaks, California, has been identified as Ian David Long.

Long, a 28-year-old former marine, is thought to have killed himself after committing the mass murder.

According to reports, the shooting began at around 11.20pm local time on Wednesday (8.20am CET on Thursday) at the Borderline Bar and Grill, 65km from Los Angeles.

The police said there had been discussions about whether Long was suffering from Post-Traumatic Stress Disorder, after an incident in April where he was described as being in a '"disturbed" and "irrational" state.

Around 200 people were inside the club, attending a college country music night, at the time. The police said the number of injured is expected to increase.

"When deputies arrived they heard shots being fired, it was an active shooter situation," Ventura County Sheriff's office spokesman Eric Buschow said.

"They entered the location and I can tell you there are multiple fatalities, multiple injuries, among the victims is one of our sheriff's deputies that responded in that initial call, as well as the shooter."

Witnesses said that panic erupted at the bar, with the suspect having fired dozens of shots.

People used chairs to break windows to escape the venue, while other hid inside toilets.

A large number of police officials descended on the bar, as investigations are under way.