By Faith Karimi, Jason Hanna and Joe Sutton, CNN

(CNN) -- A US Marines veteran opened fire at a Southern California bar late Wednesday, killing at least 12 before killing himself, and sending panicked survivors scrambling through doors and windows to escape, authorities said.

Investigators believe Ian David Long, 28, used a legally purchased Glock 21 .45-caliber handgun with an extended magazine to shoot patrons, staff and a sheriff's deputy at Borderline Bar & Grill in Thousand Oaks, and they're trying to figure out why he did it, Ventura County Sheriff Geoff Dean said Thursday morning.

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

"It's a horrific scene in there," he said earlier. "There is blood everywhere and the suspect is part of that."

Among those killed was Ventura County sheriff's Sgt. Ron Helus, who rushed in minutes after the first gunshots were reported at 11:20 p.m., Dean said.

Helus had been at the department for nearly three decades and planned to retire within the next year, the sheriff said, choking back tears.

"Ron ... gave his all and tonight, he died a hero because he went in to save lives," Dean said.

The shooting broke out during the Western-style bar's weekly line dancing and college night open to people 18 and older. People dropped to the floor and hid behind bar stools in stunned silence, witnesses said. Others jumped over chairs and broke windows to get out.

Taylor Von Molt, 21, said she was dancing when she "heard what I thought was a balloon pop."

"I heard it a couple more times, and I turned around and I saw him (the gunman)," she said. "He had ... what looked like a bandanna on the bottom on his face, sunglasses, black hoodie, dark jeans."

About 21 others were injured, many trying to escape, the sheriff's office said.

Law enforcement officials have yet to release the names of most of those who were killed or injured, and anxious friends and relatives gathered at a designated spot in the city to await word on what happened to their loved ones.

What we know about the shooting

'I started hearing these big pops' The gunman apparently arrived at the bar Wednesday night in a car, and shot an unarmed security guard who was standing outside, Dean said. Long then stepped inside, and apparently shot other security workers and employees and opened fire farther inside the club, the sheriff said.

Six off-duty law enforcement officers were in the bar when the shooting happened, Dean said. It wasn't immediately clear what happened to them.

A woman whose daughter was at the bar has said one of those officers "stood in front of her daughter, protecting her life with his own," Ventura County sheriff's Senior Deputy Julie Novak told CNN affiliate KCAL/KCBS.

Witnesses described a man clothed in black and wearing glasses as he walked onto the dance floor filled with people and began shooting.

Von Molt ran toward the nearest exit but tripped, and people kept running on top of her in a rush to escape. Eventually she got up and ran out -- and got checked at a hospital for bruises.

Von Molt said some of her friends got out, but she didn't immediately know where others were.

John Hedge, who was at the bar with his stepfather, told CNN affiliate KABC that he hit the ground after hearing three or four popping sounds.

"I started hearing these big pops. Pop, pop, pop, he said. "I look up -- the security guard is dead. Well, I don't want to say he was dead, but he was shot."

People "dog-piled on top of each other" trying to hide in silence, California Lutheran University student Teylor Whittler told KABC. Some men ran toward them and said the gunman was coming. As they escaped through the bar's back door, she heard a second round of shots.

There was no screaming, she said -- just gunfire and stampeding crowds.

Nick Steinwender, who is part of California Lutheran's student government, said his friends were inside the bar when the shooting started.

"It was chaos, people jumping out of windows, hopping over gates and just trying to get out," he told KABC. "From what I heard, the gunman started shooting at the front desk. ... Students were hiding in the attics, bathrooms and stuff like that."

The bar is near several universities, including California Lutheran, which canceled Thursday's classes in response to the shooting.

Pepperdine University said it believes "multiple Pepperdine students" also were there. A prayer service for the campus community was scheduled at a theater there at noon Thursday.

Long had previous contacts with the law, sheriff says

Deputies were at Long's home in the Newbury Park area of Thousand Oaks on Thursday morning, seeking to serve a search warrant, Dean said. Thousand Oaks is in Ventura County, about 40 miles from downtown Los Angeles.

Long was in the US Marine Corps from August 2008 to March 2013, Defense Department records show.

He subsequently was a student at California State University, Northridge, from 2013 to 2016, the school said. Long pursued a major in athletic training but did not graduate, the university's Carmen Ramos Chandler said

Law enforcement had several previous contacts with Long, including in April when officers were called to a disturbance at his home, the sheriff said.

"He was somewhat irate, acting a little irrationally," Dean said. Mental health specialists talked to him and didn't feel he qualified to be detained under the state's "5150" mental health detention law, Dean said.

One of Long's neighbors, Tom Hansen, told KABC that he had called police in April.

"I don't know exactly what he was doing -- it was like he was banging on the walls and shouting," Hansen said. "It concerned me, so I called the police."

Hansen told KABC everything seemed to go back to normal afterward and he never learned what had happened.

Long also had been a victim of battery at a different Thousand Oaks bar in January 2015, the sheriff said.

Long posted about his service with the Marines on an online forum called ShadowSpear in March 2017. He wrote he had been an infantry machine gunner, had served in Afghanistan and was an instructor in Okinawa, Japan.

Deputy killed after rushing inside

Helus, the slain sheriff's deputy, arrived at the bar 3 minutes after 911 calls came in, along with two California Highway Patrol officers, Dean said.

Helus went in because gunshots still were being fired -- and he was shot multiple times, Dean said.

A Highway Patrol officer pulled Helus out and waited for reinforcements. When more officers arrived, they found the gunman dead inside, Dean said.

Helus died at a hospital, police said. He is survived by a wife and a son.

"I don't think there's anything more heroic than what he did," a colleague, sheriff's Sgt. Eric Buschow, told CNN Thursday morning.

Helus was on the phone with his wife shortly before he responded to the shooting, Dean said.

"(He said), 'Hey, I have to handle a call. I love you, I'll talk to you later,' " the sheriff said.

Father crushed to learn his son was killed;io\

One of those who died was Cody Coffman, 22, his father said.

Cody was at the bar with friends -- and when his friends escaped, they woke his father up around 1 a.m. to tell him they didn't know if his son had gotten out.

Just before 10 a.m., the father told reporters Cody had died.

In between sobs, Jason Coffman recounted his last conversation with Cody.

"I talked to him last night before he headed out the door. First thing I said was, 'Don't drink and drive.' "

"Last thing I said was, 'Son, I love you.' "

Another victim was identified by California Lutheran as recent graduate Justin Meek, 23.

The school said he "heroically saved lives in the incident."