Ventura County Sheriff's Office Sgt Eric Buchow told the CBS affiliate in Los Angeles that there were "multiple fatalities" in the shooting and the gunman was dead inside the crowded venue in the upscale residential suburb of Thousand Oaks.
Earlier reports said a gunman barged into a large, crowded Los Angeles-area country music bar and dance hall and opened fire late Wednesday, wounding at least 11 people including a police officer, US police said.
The venue in a quiet, upscale residential suburb was hosting an event for college students, with possibly several hundred young people in attendance, Captain Garo Kuredjian of the Ventura County Sheriff's office said.
Witnesses told CNN the attacker had a large handgun, wore a black trench coat and glasses and threw smoke grenades inside the Borderline Bar and Grill in Thousand Oaks.
Kuredjian said at around 0915 GMT that the shooter was still confined inside the premises, adding that he did not know if he had been subdued or shot.
TV footage showed SWAT teams surrounding the bar, with distraught revelers milling around and using their cell phones as lights from police cars flashed.
Holden Harrah, a young man who saw the incident, cried as he told CNN that a place where he goes every week to have fun with friends had been a scene of carnage.
"A gentleman walked in the front door and shot the girl that was behind the counter. I don't know if she is alive," he said.
Police who responded to reports of a shooting rushed to the scene and engaged the gunman, said Kuredjian. A deputy sheriff is among the 11 wounded and the injury toll could rise, he added.
The Los Angeles Times quoted a law enforcement official as saying at least 30 shots had been fired.
An unnamed witness told the newspaper that someone ran into the bar around 11:30 pm and started shooting what looked to be a black pistol.
"He shot a lot, at least 30 times. I could still hear gunshots after everyone left," the Times quoted the man as saying.