A gunman opened fire inside a crowded country-music bar in Southern California, killing 12 people, including a sheriff's deputy who had rushed inside to confront the shooter, authorities said. The gunman was later found dead inside.

The gunfire erupted as people were line dancing on "College Country Night" at the Borderline Bar & Grill, a popular nightspot in Thousand Oaks, California, witnesses said.

The gunman has been identified as Ian David Long, a 28-year-old former US Marine who may have been suffering from PTSD, the Ventura County sheriff said.

A victim is carried from the scene of a shooting after a gunman opened fire in a bar killing 12 people. Photo / AP
A victim is carried from the scene of a shooting after a gunman opened fire in a bar killing 12 people. Photo / AP

Long, who lived in nearby Newbury Park, tossed smoke bombs and unleashed a hail of bullets from a .45-calibre Glock 21 fitted with an extended magazine at the bar in Thousand Oaks, about 65km north of Los Angeles, Sheriff Geoff Dean told reporters.

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Sheriff Dean said his officers had several interactions with Long, including an April call to his home, according to the New York Post.

"We've had several contacts with Mr. Long over the years, minor events, a traffic collision," he said.

"In April of this year, deputies were called to his house for a subject disturbing. They went to the house, they talked to him. He was somewhat irate. Acting a little irrationally."

Mental health professionals sent to evaluate him concluded he did not need to be taken into custody, he said.

Long also was the victim of an assault at a local bar in 2015, the sheriff said.

Among the dead was Ron Helus, a veteran sergeant in the Ventura County Sheriff's Office who was mortally wounded when he responded to the incident northwest of Los Angeles just minutes after 911 calls began flooding in, authorities said.

The only weapon recovered by early Thursday was a handgun, Dean said.

"It's a horrific scene in there," Dean told reporters. "There's blood everywhere. The suspect is part of that."

Dean said he had no reason to believe that there was a link to terrorism, "but we certainly will look at that option."

Helus, the deputy who died at the hospital about an hour after sustaining his injury, was a 29-year veteran of the force and was expecting to retire soon. He was survived by a wife and a son. "He died a hero," Dean said, his voice cracking, "because he went in to save lives."

President Donald Trump said on Twitter that he had been briefed on the shooting, and praised the "great bravery shown by police," adding: "God bless all of the victims and families of the victims."

Sgt. Ron Helus, who was killed when he responded to the 911 call. Photo / AP
Sgt. Ron Helus, who was killed when he responded to the 911 call. Photo / AP

In addition to the dead, Dean estimated that there were upward of 10 to 12 shooting victims who were "rescued from the scene and taken to local hospitals." Additional victims with minor injuries fled the scene on their own, he added.

"I know there are parents wondering, 'Oh my gosh, was one of my children in there? Or was it my sister or my brother?'" he said.

Dean said the mass shooting, at a bar packed with students from nearby colleges, "is part of the horrors that are happening in our country and everywhere, and I think it's impossible to put any logic or any sense to the senseless."

Reports of a shooter came in about 11.20pm Pacific time Wednesday, and authorities arrived on the scene at 11.22pm, Dean said.

People walk away from the scene where a gunman opened fire inside a country dance bar crowded with hundreds of people inside. Photo / AP
People walk away from the scene where a gunman opened fire inside a country dance bar crowded with hundreds of people inside. Photo / AP

Helus, the sheriff's sergeant, entered the venue with a California Highway Patrol officer several minutes later, at which point Helus was struck "multiple times," Dean said. The CHP officer stepped back and secured the perimeter.

The gunfire briefly quieted, the sheriff said, as people scrambled to hide in restrooms and in attics.

"It was sheer panic," said Teylor Whittler, 19, who was inside the venue at the time. "Everyone ran and dropped as fast as they could."

She said she ran to the back door, where people crowded during a pause in the gunfire. "And then all, of a sudden, a couple of guys started running to the back door and said, 'Get up he's coming.'"

The shooting unfolded just 13 months after 58 people were killed at a country music festival in Las Vegas, when a lone gunman opened fire from the 32nd floor of the Mandalay Bay Resort and Casino.

The Borderline Bar describes itself as Ventura County's largest country dance hall and live music venue. With a dance floor covering about 2,500 square feet, it is open until 2 a.m. five days a week.

Authorities said more than 100 were inside at the time of the shooting. Scores of colleges lie within a 20-mile radius of the bar, including Pepperdine University, California Lutheran University and Moorpark College.

Pepperdine officials said that multiple students from the school were at the bar during the shooting. "University officials have been working on site with law enforcement to confirm their safety and well-being, and our prayers and condolences are with everyone impacted by this senseless act of violence," the school said.

Claire Gietzen told an ABC affiliate that she ran behind the bar when gunshots broke out, but then joined a man who pulled down a ladder leading into the attic.

Holden Harrah, 21, (R) who witnessed the shooting hugs family and friends. Photo / Getty Images
Holden Harrah, 21, (R) who witnessed the shooting hugs family and friends. Photo / Getty Images

"He motioned for me to follow him. I thought that was the best option at the time," she said. "[We heard] gunshot after gunshot. I heard glass breaking. I heard commotion and screaming. … We kept thinking it would stop for a while, that we were okay, and then it would start up again."

Special weapons and tactics teams arrived on the scene a short time later, along with the FBI. Video from the scene showed bar-goers rushing injured people to medical response vehicles.

When the authorities re entered the nightclub, they found the suspect - whom they believe to be the lone gunman - dead inside, Dean said.

"We don't know who shot the deputy," he added. "We don't know who shot anybody at this point."

He said that authorities have not yet identified the suspect.

Matt Wennerstrom, a regular at the bar, described the shooter as a "tall figure," over 6 feet, wearing "all dark clothing." He said he saw the gunman open fire on employees working at the front of the bar.

"At that point I grabbed as many people around me as I could and pulled them down underneath the pool table that we were closest to until he ran out of bullets for that magazine and had to reload," the eyewitness said.

During the pause, Wennerstrom, 20, said he and others threw bar stools through a window and helped people escape. He told ABC he was able to push "30 or 35 people through that window."

Other witnesses said the gunman was wearing black, including a hoodie.

A man and his stepfather interviewed by ABC said they heard about 12 shots before they were able to exit through the front door of the nightclub.

"He fired the first shot," the stepfather said. "And I knew it was real. My son thought it was a joke, so I pulled him down and got some cover. I looked up, and he was moving to the right. He shot the front doorman, who was just a young man. Then he shot the cashier, just a young girl."

Multiple eyewitnesses described seeing smoke, possibly from smoke bombs.

Nellie Wong cries as Chyann Worrell holds her and Erika Sigman (R) were all inside when shooting started inside the Borderline Bar & Grill . Photo / Getty Images
Nellie Wong cries as Chyann Worrell holds her and Erika Sigman (R) were all inside when shooting started inside the Borderline Bar & Grill . Photo / Getty Images

Rochelle Hammons, 24, told The Post that she heard four shots before she was able to flee.

"All of a sudden we heard four shots, you know, 'bang, bang, bang, bang.' Everyone got down on the floor. Everyone ducked and covered each other," she said. "As everyone crouched down on the floor, I figured that my only chance would be to run out to the nearest exit. I saw the nearest exit, and I ran out as fast as I could."

From inside her car, she saw the first police officer arrive, she said. She rolled down her window and told him there was an active shooter inside.

"You've got to hurry, you've got to get in there," she urged him.

The massacre was the deadliest mass shooting in the US since 17 students and teachers were slain at a Parkland, Florida, high school nine months ago.

It also came less than two weeks after a gunman killed 11 people at a synagogue in Pittsburgh. That, it turn, closely followed the series of pipe bombs mailed to critics of President Donald Trump