The Times reports that there were multiple reports of shots fired and people hit.
He did say that it was still "a very active scene".
Another witness said he was at the front door of the bar when the shooter began firing. The security guard was dead and the man had a handgun.
"The gunman was throwing smoke grenades all over the place", a different witness told KABC.
He said the gunman had a beard, wore a hat, had a black jacket and may have had glasses. Threw in smoke to confuse people and opened fire on the dancefloor.
Officers from the Ventura County Sheriff's Office and California Highway Patrol said about 30 shots were fired and several people were injured.
Police have described it as a "mass casualty incident" and helicopters are swirling overhead.
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Officers escorted large groups of people to safety as they left the area.
A SWAT team have been pictured surrounding the venue in their formations.
It's thought that a man opened fire at the bar with a semi-automatic gun at about 11.20pm.
They said that soon after the gunman opened fire, victims were running to a nearby petrol station to get help.
Borderline is a country music bar that hosts live music events.
According to the bar's website, a "College Country" night was going on at the time of the shooting.