Harrowing video of the Borderline Bar shooting has emerged, as it is revealed that one of the many Las Vegas shooting survivors present in the bar was among the 12 people slain.

The sickening video shows ex-Marine Ian Michael Long, 28, stalking through the Borderline Bar and Grill in Thousand Oaks, California late Wednesday, firing off shot after shot.

Haze from smoke grenades that Long used to disorient the victims fills the air, and the bar is eerily silent except for the sound of shattering glass and the shots from a Glock .45-caliber handgun with an extended magazine, the Daily Mail reported.

In this image taken from video a victim is carried from the scene of a shooting. Photo / AP
In this image taken from video a victim is carried from the scene of a shooting. Photo / AP

What appears to be the gunman can be glimpsed for an instant before the witness holding the camera manages to bolt from the bar out a side door.

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"I looked him in his eyes while he killed my friends," wrote Dallas Knapp, the witness who shot the video.

"I hope he rots in hell for eternity. The guy you see is the murderer and the shots are him shooting people trying to get out the windows."

Telemachus Orfanos was among those killed before the shooter turned the gun on himself.

Like others who gathered at Borderline for country music night regularly, Orfanos was a survivor of the October 1, 2017 shooting massacre at the Route 91 Harvest Festival in Las Vegas, where 58 died and hundreds were injured.

Orfanos, an Eagle Scout who had served in the Navy, was working at the bar when he was killed.

Other victims inside the bar included: Sean Adler, 48; Cody Coffman 22; Blake Dingman, 23; Jake Dunham, 21; Alaina Housley, 18; Justin Meek, 23; Daniel Manrique; Kristina Morisette; Telemachus Orfanos; and Noel Sparks.

Long then shot sheriff's sergeant Ron Helus, a 54-year-old, 29-year veteran who was one of the first on the scene. He died in the hospital of multiple gunshot wounds.

Between 10 and 15 people remain in hospital, some with severe injuries. The mayor has appealed for blood donations, and hundreds responded by turning out to donate.

Survivors used bar stools to smash windows to climb out of and some hid beneath pool tables. One woman ran into the kitchen and was told by staff to climb a ladder into the attic.

According to survivors, Long was dressed in all-black, wearing a baseball cap, sunglasses and a mask covering the bottom part of his face.

Long was a machine gunner in the Marines Corps and served until 2013. He was deployed once to Afghanistan and received 10, standard-issue medals during his service. Photo / Supplied
Long was a machine gunner in the Marines Corps and served until 2013. He was deployed once to Afghanistan and received 10, standard-issue medals during his service. Photo / Supplied

Before SWAT teams entered the building, Long took his own life in an office inside. He used a legally purchased .45 caliber handgun to carry out the attack and had modified its magazine so it could hold more rounds.

Long served in the Marines as a machine gunner for five years between 2008 and 2013, during which time he was decorated with 10 medals and toured Afghanistan.

In a cruel twist of fate, dozens of people who survived the Las Vegas shooting were inside the Borderline bar when Long opened fire.

"It's the second time in about a year and a month that this has happened," Nick Champion told OnSceneTV. "I was in the Las Vegas Route 91 mass shooting, as well as probably 50 or 60 others who were in the building at the same time as me tonight."

Champion and his friends were inside country music bar Borderline Bar and Grill for student night Wednesday, when ex-marine Ian Long, 29, stormed in, shooting dead 12 before turning the gun on himself.

He and other Las Vegas survivors considered Borderline a "place of healing" and gathered at the same bar for a memorial on a student night earlier this year. The group of survivors posed in front of a Borderline sign while holding a Route 91 Harvest banner.

"It's a big thing for us. We all are a big family, and unfortunately, this family got hit twice," Champion said.

The Las Vegas shooting became the worst in modern American history when gunman Stephen Paddock opened fire through the windows of his suite at the Mandalay Bay hotel which overlooked the country music festival - an event that attracted many of the same audience as those at Borderline.

"A lot of people in the Route 91 situation go here,' Chandler Gunn told the Los Angeles Times. 'There's people that live a whole lifetime without seeing this, and then there's people that have seen it twice."

Paddock's motives in the Las Vegas shooting were never determined by investigators, while in California the probe into what motivated Long to senselessly kill 12 is just beginning.

Survivors from the Borderline shooting said the gunman looked 'like he knew what he was doing' as he repeatedly fired his gun without saying a word.

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

In April this year, deputies were called to the home he shared with his mother after neighbors heard loud crashes coming from inside.

He was acting "irrationally" and was "irate" so police called in their mental health specialist but they cleared him, deciding against having him committed.

Neighbours told DailyMail.com on Thursday that his mother Colleen was "terrified" he was going to harm himself or others, and that they believed Long suffered from PTSD - though some said that Long's behavior was odd even before his military service.

Long served in the Marine Corps for five years between 2008 and 2013 and he toured Afghanistan. He was a corporal at the time of his honorable discharge.

During his time in the Marines, he was decorated with numerous awards including two Navy Meritorious Unit Commendations, a Combat Action Ribbon, the Marine Corps Good Conduct Medal, two Sea Service Deployment Ribbons, the Afghanistan Campaign Medal, the Global War on Terrorism Service Medal; the National Defense Service Medal and the NATO Medal-ISAF Afghanistan.

All are relatively common decorations.

This undated photo provided by the Ventura County Sheriff's Department shows Sheriff's Sgt. Ron Helus, who was killed. Photo / AP
This undated photo provided by the Ventura County Sheriff's Department shows Sheriff's Sgt. Ron Helus, who was killed. Photo / AP

A spokesman for the Marines declined to give information surrounding his discharge in 2013, telling DailyMail.com that they were bound by privacy laws and unable to reveal what prompted him to leave.

They did offer condolences to the victims of Wednesday night's shooting.

Curtis Kellog, a friend who he served with, said Long had a 'great sense of humor' and was excited to return to southern California after leaving the military.

"He had a great sense of humour and like most Marines who have seen combat it could get dark at times, just like all of us.

"He was excited to get out so he could go back home, ride his motorcycle again and finish school," he told Click 2 Houston.

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

After leaving the Marines, Long took a job as an athletic trainer with California State University Northridge, where he also attended classes as a student until dropping out in 2016.

"I found out a little too late that just wasn't the job for me. Maybe the ego got the better of me but it took only one time for a 19 year old D-2 athlete to talk down to me and tell me how to do my job that I realised this wasn't the career I wanted to head," he said of his departure in a March 2017 post that was uncovered by CNN on the forum Shadowspear.

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

Blake Winnett, who claims to have shared an apartment with him in 2014 while he was a student at CSUN, told The New York Post that he was a "loner" who danced alone in their garage.

"He didn't want to help anyone do anything. He was just lazy I guess," he claimed, adding that he once responded:

"That's not my f****** job" when Winnett asked him to take out the trash.

A group of civilians, law enforcement and fire personnel, look on from an overpass as a motorcade with the body of Ventura County Sheriff's Sgt. Ron Helus passes. Photo / AP
A group of civilians, law enforcement and fire personnel, look on from an overpass as a motorcade with the body of Ventura County Sheriff's Sgt. Ron Helus passes. Photo / AP

"He wasn't violent but he was mean. He would go to the gym and then he would, I guess, try to learn dance moves or something.

"He would close the garage and be playing music and dancing in there, like sweating.

"I would open the garage and would be like, 'What are you doing?'" he said.

Long dropped out of the university in 2016 after three years.

He had been studying athletic training. Sometime after dropping out of college, he returned to his mother's home in Newbury Park.

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

One neighbor told DailyMail.com that his mother "lived in fear" that something would happen to him.

Another, Julie Hanson, who lives with her husband next door, told the AP that she always found Long odd and disrespectful, even before his time with the Marines.

Long's mother Colleenn Long, 61, was spotted by DailyMail.com being interviewed by FBI agents on Thursday.

She emerged from her house around 1.45pm and was shepherded into another red truck resembling the one the shooter used to drive to the Borderline Bar & Grill where he carried out the massacre. She and several officers left the house in the smart suburb of Newbury Park in a convoy of three trucks.

In addition to the 12 he killed inside the Borderline Bar and Grill on Wednesday, '10 to 15' victims were injured.

A GoFundMe page has been set up to help cover their medical bills. They have raised $5,000 of the $50,000 they hope to get.

In a Facebook post on Thursday afternoon, the bar said: 'The Borderline Family sends our love, prayers and deepest condolences to the victims and their families in the wake of this horrific tragedy.

"Thousand Oaks is more than the community we do business in but rather our brothers, sisters, dads, mommas, friends, family and neighbours.

"We love you and together we will stand strong."

Borderline Bar and Grill is described as the county's "Largest Country Dance Hall & Live Music Venue" and boasts of 2500 sqft of open dance floor, with pool tables, games and flat-screen TVs.

Savannah Stafseth, who was outside on the patio when the shooting started, told the LA Times: "There are no words. Those are my people. It's just not fair. It's not fair. All these people after Route 91. It's not fair."

Thousand Oaks is known as an area with low crime and is considered one of the safest cities in America.

Another witness told ABC7 California TV station: "A lot of my friends survived Route 91. If they survived that, they will survive this."