Terrified patrons scrambled for the exits after the gunman started shooting a handgun in the Borderline Bar and Grill around 11:20 pm on Wednesday before killing himself, they said.

"They ran out of the back doors, they broke windows. They went through windows. They hid up in the attic, they hid in the bathroom," Ventura County Sheriff Geoff Dean said.

Dean identified the suspect as Ian David Long, a Marine Corps veteran and resident of Thousand Oaks, an upscale city northwest of Los Angeles.

According to the Pentagon, Long served in the Marine Corps from August 2008 to March 2013 and was a machine gunner.

He attained the rank of corporal and saw combat while deployed in Afghanistan from November 2010 to June 2011.

"He had perfect form," bar patron Teylor Whittler told Fox News. "He looked like he knew what he was doing."

Long was found dead in an office at the bar by police and is believed to have shot himself, Dean said.

Authorities so far had no indication of a motive, or of any connection to terrorism.

"We have no idea what the motive was at this time," Dean said.

"Obviously he had something going on in his head that caused him to do something like this," he said. "He had some kind of issues."

- 'Like hell' -

The sheriff said Long drove to the bar and shot an unarmed security guard who was standing outside before opening fire at random inside.

Dean described the scene as "like hell."

He said his department had "several contacts" with Long over the years, for minor incidents including a traffic collision, and in 2015 when he was beaten up at a local bar.

In April this year, deputies were called to his house for a disturbance and found him "acting a little irrationally."

"They felt he might be suffering from PTSD, the fact he was a veteran and had been in the corps," Dean said.

"They called out our crisis intervention team, our mental health specialists who met with him, talked to him, and cleared him."

Dean said Long was armed with a single handgun, a .45 caliber Glock 21, which he purchased legally.

Among those killed was a three-decade veteran of the sheriff's department, 54-year-old Sergeant Ron Helus, who was married, had a grown son and was close to retirement.

"He went in there to save people and made the ultimate sacrifice," Dean said.

He said Helus was on the phone with his wife when he got the report of the shooting and he told her "'Hey, I have to go. I love you. I'll talk to you later.'"

Helus and a highway patrol officer exchanged gunfire with Long, he said, and the sergeant was shot "several times."

- 'Sounded like firecrackers' -

Around 15 people were also injured, the sheriff said, including several who were hurt "jumping out of windows, diving under tables."

Matt Wennerstron, a 20-year-old college student, said the shooter apparently had a 10-15 round magazine on his gun.

"When (he) started to reload, that's when we got people out of there and I didn't look back," Wennerstron said.

He said he and others smashed their way out of the bar onto a balcony and then jumped down to safety.

Jasmin Alexander, who was with around 15 friends at the bar, said there was chaos and confusion inside when shots rang out.

"We were just at the bar, having fun, dancing," she told reporters.

"All of a sudden we heard the 'bang, bang' of the gunshots and it just started going crazy and people were pushing.

"We didn't take it seriously at first... because it sounded like firecrackers and everyone just dropped down to the floor."

TV footage showed SWAT teams surrounding the bar, with distraught revelers milling around and using their cell phones as lights from police cars flashed.

President Donald Trump tweeted that he had "been fully briefed on the terrible shooting," the latest chapter in America's epidemic of gun violence that came just 10 days after a gunman killed 11 worshippers at a synagogue in Pittsburgh.

Trump ordered the US flag at the White House and other government buildings to be flown at half-staff.

Last year, a country music festival in Las Vegas was the scene of the worst mass shooting in modern US history. A gunman shooting from the 32nd floor of a hotel and casino with high power weapons killed 58 people.