Ian David Long, 28, is believed to have been the gunman in Wednesday night's shooting at the Borderline Bar & Grill in Thousand Oaks, California, Ventura County Sheriff Geoff Dean said.
Authorities found the gunman dead inside an office adjacent to the bar's entryway, Dean said. "We believe he shot himself."
The gunman used a Glock 21 .45-caliber handgun designed to hold 10 rounds plus one in the chamber, but the gun had an extended magazine, the sheriff said. Investigators didn't immediately know how many rounds the magazine could hold, he said.
Long was a US Marine Corps veteran, Dean said.
"We have no idea what the motive was at this point."
Deputies are at Long's home, seeking to serve a search warrant, Dean said.
Investigators believe Long arrived Wednesday night at the bar in a car, shot a security guard who was standing outside and then stepped inside. It appears that Long then shot other security workers and employees inside and opened fire further inside the club, Dean 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.
Long also had been a victim of battery at a different Thousand Oaks bar in January 2015, the sheriff said.
[Original story, published at 9:27 a.m. ET]
A gunman burst into a packed bar in Southern California and opened fire on people line dancing, killing at least 12 before he was found dead at the scene, authorities said.
Ventura County sheriff's Sgt. Ron Helus was among those killed Wednesday night at the Borderline Bar & Grill in Thousand Oaks, authorities said. At least 10 to 15 others were injured, Ventura County Sheriff Geoff Dean said.
Helus, 54, 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 Western-style bar is near several colleges, and it was hosting a college night for country line dancing, open to anyone 18 or older, witnesses said.
The gunman opened fire around 11:20 p.m., police said. Witnesses described a man clothed in black and wearing glasses as he walked onto the dance floor filled with people.
The suspect was found dead inside the bar, Dean said. It's unclear whether he took his own life. The motive of the attack also is not clear, the sheriff said, and authorities are looking at all possibilities, including it being terror-related.
"It's a horrific scene in there," he said. "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."
'I started hearing these big pops'
The shooting started while patrons were line dancing -- and 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 line 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."
She 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."
California Lutheran University student Teylor Whittler told KABC that after she heard a shot, she "turned around and I saw him (the gunman) shoot a couple more times."
People "dog-piled on top of each other" trying to hide in silence, she said. 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 the affiliate. "From what I heard, the gunman started shooting at the front desk. ... Students were hiding in the attics, bathrooms and stuff like that."
Pepperdine University said it's working to identify some of its students who were at the bar.
"The University offers its deepest condolences and is praying for everyone involved in tonight's tragic events," it tweeted.
Deputy killed after rushing inside, sheriff says
Dean, the sheriff, said Helus and two California Highway Patrol officers arrived at the bar 3 minutes after 911 calls came in.
Helus went in because gunshots still were being fired, Dean said.
"Upon going through the front door, (Helus) was struck multiple times with gunfire," 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.
Authorities are investigating reports that the gunman used a smoke bomb but said they had not found anything to support this.
"We have called in our bomb team to go through and a couple of bomb dogs here that are checking the area," Dean said.
Father fears son 'ran to the gunman'
Jason Coffman was one of the parents anxiously awaiting news Thursday morning about those inside the bar.
His son, Cody, 22, was there with friends -- and while his friends got out, they didn't know where Cody was, he told CNN and HLN.
The father told CNN he can track his son's cell phone -- and the tracking indicates it still was in the bar Thursday morning.
"It's at the club. It's not moving. That's the problem," said Coffman, who was awaiting information at a center for relatives.
"I am very emotional right now," he said. "This is my firstborn son and it's tough."
The father said Cody's friends woke him up at 1 a.m. to tell him about the shooting.
"I'm afraid that Cody ran to the gunman instead of away from the gunman. That's the kind of boy Cody is," he said.
City ranked one of the safest, mayor says
Thousand Oaks is in Ventura County, about 40 miles from downtown Los Angeles. The city "is considered one of the safest cities in the country," according to Mayor Andy Fox.
"We are consistently ranked one of the highest with respect to the lowest crime rate per capita. We're proud of that," Fox said.
"The reality is that these types of incidents can happen really at any place, at any time, even in communities that are considered extremely safe."