A shirtless man and two others carry an injured person out of the Borderline Bar and Grill in Thousand Oaks, California, on Wednesday night after a gunman opened fire at 11.20pm 

By Connor Boyd For Dailymail.com

A gunman opened fire during a student night at a country music bar in the city of Thousand Oaks,California, killing 12 people and injuring several before turning the gun on himself.

The shooter, who has not yet been named, stormed the Borderline Bar and Grill, wearing a black trenchcoat, black bandanna and armed with a pistol equipped with an extended magazine and smoke grenades, and began targeting people as young as 18.

Among those killed was Ron Helus, a Ventura County sheriff sergeant who was killed after responding to the scene.  The ‘hardworking’ officer, who was due to retire this year, had 29 years of experience in the force and leaves behind a wife and son.

An additional ’10 to 15′ victims were injured during the mass shooting, including the bouncer, a young female cashier and college students.

The gunman shot himself before police got inside. He was dressed in all-black, wearing a baseball cap, sunglasses and a mask covering the bottom part of his face.

He walked up to the entrance at 11.20pm and shot the bouncer before making his way into the venue. He then opened fire on a young female cashier before throwing smoke grenades onto the dance floor and letting off rounds into the crowd.

First responders from the Ventura County Sheriff’s Office and Highway Patrol arrived at 11.23pm and got into a gunfight with the suspect.

I was just dancing normally and I heard what I thought was a balloon pop. I was confused then I heard it a couple more times then I turned around and I saw him.

‘I ran to the nearest exit and tripped and fell on the way I knocked on as many doors as I could until someone would answer me and finally someone did.

‘It was awful,’ Taylor Von Molt, a promoter for the club, told CNN.

She said the gunman’s mask looked like a bandanna and was covering the bottom of his face.

‘He had a black mask, it looked like a bandanna, on the bottom of his face, sunglasses dark hoodie, dark jeans. He was kind of bigger, more burly. Scary,’ the 21-year-old said.

An injured man who appears to have been shot is rushed to safety at the Borderline Bar and Grill

More than 100 people, many of whom were students (pictured) as young as 18, were in the bar when the gunman opened fire

A young reveler is in a state of shock after witnesses a gunman fire at least 30 rounds at a crowd of country music fans

Several minutes after the gunman opened fire, a SWAT team arrived and found 11 victims dead inside.

‘It’s a horrific scene in there, there is blood everywhere and the suspect is part of that, and I didn’t want to get that close and disturb the scene and possibly disturb the investigation,’ Ventura County Sheriff Geoff Dean said afterwards.

He said Helus was a ‘hero’ who died to protect others.

‘Ron was a hardworking dedicated sheriff’s sergeant. He was totally committed, he gave his all and tonight, as I told his wife, he died a hero; he went into save lives, to save other people,’ he said.

There were 100 people inside the bar when the gunman opened fire, police said, and the number of casualties is ‘very likely’ to go up.

One witness told ABC7: ‘I was at the front door talking to my stepdad. I heard these big pops.

‘There were three or four and I hit the ground. The security guard was dead and the man had a handgun.

‘He threw in smoke bombs and he kept firing. People threw chairs out of the windows to try to get away.

‘The gunman had glasses and a black jacket. He had a big hand gun.’

Another witness said: ‘This maniac came in. Threw in smoke to confuse people and opened fire on the dancefloor.

‘He’s taken many young lives.

Witnesses reported a horrifying scene as gunfire echoed through the club and those inside ran for cover, in some cases breaking windows with chairs and other pieces of furniture to get out of the building.

The bar is popular with college kids because it allows under 21s in on Wednesday nights.

People were hiding from the shooter in bathrooms and attic spaces, while others who managed to escape fled to nearby gas stations for medical help.

Local hospitals have been inundated with victims who’ve transported themselves to emergency rooms with injuries seeking medical help.

One witness, who was at the bar with his step-son, described the sequence of events. He said:  ‘He shot the front doorman, who was just a young man.

‘Then he shot the cashier, just a young girl. Then he started moving to the right. He wasn’t looking at us. Then he went into the office, where all the cash and stuff is.

‘He didn’t say anything at all. He just started shooting.’