It was just before 1am when Jason Coffman was told by some of his son's friends that there had been a shooting at Borderline Bar, a venue he knew his 22-year-old by Cody was at.

Fearing the worst, Mr Coffman tracked his son's phone and realised it was sitting motionless inside the club.

"It's there," Mr Coffman told CNN. "It's not moving, that's the problem."

Cody Coffman planned to join the military before his death. Photo / via Facebook
Cody Coffman planned to join the military before his death. Photo / via Facebook

Hours later, Cody was identified as one of the 12 victims who died when a gunman opened fire inside the club that had been hosting a college night.

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Speaking to reporters, Mr Coffman sobbed as his worst fears came true.

"They found him," he told reporters. "I've been here fighting for him all morning long."

"Oh Cody, I love you son. This is a heart I will never get back. I cannot believe that it's happened in my family.

"I don't know how to console or what to say to the other people who are going to go through the same thing as I am. I am so sorry for them. I am speechless and heartbroken."

Before hearing of his son's death, Mr Coffman had been frantically driving to hospitals around Thousand Oaks in the US state of California begging for information.

Eventually, he was told to wait at a centre with other parents of unaccounted for children.

"I am not at the hospital, I'm at the teen centre where they tell the parents to go to basically wait for information," Mr Coffman told CNN.

"I have not got any information, I am in the dark right now and it's actually tearing me up."

In this image taken from video a victim is treated near the scene of the shooting. Photo / AP
In this image taken from video a victim is treated near the scene of the shooting. Photo / AP
This 2017 photo from the California Department of Motor Vehicles shows Ian David Long. Photo / AP
This 2017 photo from the California Department of Motor Vehicles shows Ian David Long. Photo / AP

Mr Coffman remembered the last conversation he had with his 22-year-old son and said he knew Cody would've been trying to help people inside the bar.

"The first thing I said was 'Please don't drink and drive.' The last thing I said was 'Son, I love you'," Mr Coffman told reporters.

"Some of his girlfriends got out but they didn't know where Cody was, I'm sure he was there helping, shielding anybody that he could.

"I know he went there with a few of his girlfriends so I am hoping that he's OK."

Cody, a star athlete, was hoping to join the military before his death and had been talking to army recruiters.

Cody was the second victim identified in America's latest mass shooting. The first was Ventura County sheriff's Sergeant Ron Helus, who was shot multiple times in an exchange of gunfire with the attacker.

Law enforcement and fire personnel salute from an overpass as a motorcade with the body of Ventura County Sheriff's Sgt. Ron Helus passes by. Photo / AP
Law enforcement and fire personnel salute from an overpass as a motorcade with the body of Ventura County Sheriff's Sgt. Ron Helus passes by. Photo / AP
Colleen Long, mother of shooting suspect David Ian Long, leaves her house in Newbury Park. Photo / AP
Colleen Long, mother of shooting suspect David Ian Long, leaves her house in Newbury Park. Photo / AP

A 23-year-old California Lutheran University student has also been identified as another victim.

The university said in a statement that the family of recent graduate Justin Meek said he was among the dead.

The bloodshed was the latest in what seems to be a never-ending string of mass shootings that are happening with terrifying frequency across the United States.

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

It also came less than two weeks after a gunman massacred 11 people at a synagogue in Pittsburgh.

The Borderline Bar shooting left 12 dead, including the police officer who responded to first reports.

Using a smoke bomb and a handgun, the hooded Marine combat veteran dressed all in black opened fire during college night at the country music bar in Southern California, killing 12 people and sending hundreds fleeing in terror before apparently taking his own life, authorities said.

The killer was identified as 28-year-old Ian David Long, a former machine gunner and veteran of the war in Afghanistan who was interviewed by police at his home last spring after an episode of agitated behaviour that they were told might be post-traumatic stress disorder.

Screaming in fear, patrons rushed for the exits, dived under tables and used barstools to smash second-floor windows and jump to safety as gunfire reverberated through the Borderline Bar & Grill, a hangout popular with students from nearby California Lutheran University.

The dead included 11 people inside the bar and a veteran sheriff's sergeant who was the first officer inside the door, Ventura County Sheriff Geoff Dean said.

"It's a horrific scene in there," Dean said in the parking lot. "There's blood everywhere."