Investigators are desperately searching for a motive after at least 13 people were left dead when a US war veteran used a smoke bomb and a modified handgun in a methodical shooting rampage at a Californian bar.

More than 15 people were wounded in the attack on Thursday (Wednesday night local time) at the Borderline Bar & Grill country music venue in Ventura County near Los Angeles, a popular hangout for students from nearby California Lutheran University.

The suspected shooter, named as 28-year-old former marine Ian David Long, was among the dead after taking his own life, police said.

Police said Long was wearing a hood and dressed in black when he opened fire in the packed bar, sending hundreds of patrons running for their lives.

Screaming in fear, patrons rushed for the exits, ducked under tables and hurled barstools to smash second-floor windows and jump to safety as the gunfire erupted, the Associated Press reported.

“I dropped to the floor,” Sarah Rose DeSon told US broadcaster ABC. “A friend yelled, ‘Everybody down!’ We were hiding behind tables trying to keep ourselves covered.”

Police said at least 200 people were inside for a line-dancing night at the bar.

Many of the Borderline bar’s regular patrons were thought to be survivors of the Las Vegas massacre at the Route 91 country music festival last year.

“A lot of people in the Route 91 situation go here,” said Chandler Gunn, 23, who went to the bar after the shooting, knowing some of his friends were inside.

Among the 12 people killed inside the bar was veteran Sheriff’s Sergeant Ron Helus, who was the first officer inside the door, Ventura County Sheriff Geoff Dean said.

Sergeant Helus was a 29-year police veteran. Photo: Ventura County Sheriff’s office

“It’s a horrific scene in there,” Sheriff Dean said. “There is blood everywhere and the suspect is part of that.”

The Ventura County massacre was the third mass shooting in the US in under two weeks, six days after the death of two women at a yoga class in Tallahassee, Florida and 12 days after a gunman killed 11 worshippers at a synagogue in Pittsburgh.

It was the nation’s deadliest shooting attack since 17 students and teachers were killed at a Parkland, Florida, high school nine months ago.

Sheriff Dean told reporters that Long walked up to the bar and shot a security guard standing outside before entering the venue and shooting other security guards and employees.

He then opened fire inside the bar, the sheriff said.

Sheriff Dean said the gun used by the shooter was designed to hold 10 rounds, but was equipped with an illegal extended magazine.

"The shots kept going and going and going, we just heard people say run. We ran as fast as we could" witnesses of a mass shooting in Southern California describe the moment they came under attack, after a gunman opened fire in a bar – https://t.co/HmSWxhUqg8 pic.twitter.com/fzK1mRk50W

— ABC News (@abcnews) November 8, 2018

Witnesses spoke of being thrown by friends out of windows, or remaining still on the floor for up to 90 minutes to flee the scene.

Officers said victims ran to a nearby petrol station to get medical help.

At least four ambulances were called to the scene, where officers were told at least 30 rounds of shots were fired from a semi-automatic gun.

Gunfire continued as Long engaged in a shootout with Ventura County Sheriff’s Office police and and California Highway Patrol officers.

VIDEO: KABC-TV helicopter image shows person loaded into ambulance, rescue workers conducting CPR, during active shooter situation in Southern California https://t.co/KRlguIpI2p pic.twitter.com/DAUWov5YrS

— Matthew Keys (@MatthewKeysLive) November 8, 2018

The AP reported that Dean was a former machine gunner and decorated veteran of the war in Afghanistan who was interviewed by police at his home last year after an episode of “agitated behaviour” that they were told might be post-traumatic stress disorder.

After serving in Afghanistan from 2008 to 2013, he moved to southern California to pursue a degree in sports medicine before leaving his studies in 2016, CNN reported.

Many families of those who were at the bar were still awaiting word on missing loved ones as the Ventura County Sheriff’s Office worked through notifications.

“It’s a slow and methodical process,” Sheriff Deputy Bill Hutton said.

“Everyone here wants to know – family, friends, relatives. We’re doing everything we can to get information (out to them).”