The shooting happened around 11:20 p.m. Wednesday at Borderline Bar & Grill at 99 Rolling Oaks Drive. Ventura County sheriff's officials said deputies initially responded to multiple reports of shots fired at the popular establishment.
One survivor was wounded by gunfire, and as many as 15 others suffered minor injuries from jumping out windows or diving under tables, authorities said.
Sheriff Geoff Dean said the gunman immediately shot a security guard after entering the bar, turned to the right and shot several other security officials, employees and patrons.
Terrified patrons flee from gunfire
Witness John Hedge, of Moorpark, said he saw the suspect throwing smoke bombs into the front of the restaurant, located off Moorpark Road near the 101 Freeway. He also said he saw one security guard get shot.
"I was at the front door and I was talking to my stepdad. I just started hearing these big pops. Pop, pop, pop. There was probably three or four, I hit the ground. I look up, the security guard is dead. Well, I don't want to say he was dead, but he was shot.
"He was down. The gunman was throwing smoke grenades all over the place. I saw him point to the back of the cash register...and he just kept firing. I ran out the front door," he said.
Hedge added that there may have been about 12 shots by the time he got out of the door. He described the gunman as having a beard and wearing a hat, black jacket and possibly glasses.
WATCH: FBI working to determine motive for Thousand Oaks mass shooting
"I thought it was a joke when the shots started firing. I know people there. I hope everybody's O.K. I don't know how I didn't get shot," the man said.
Tim, Hedge's stepfather, who was in tears, said he was next to the entrance about to leave after playing pool when smoke came into the room.
"He fired the first shot. I knew it was live. I knew it was real. My son thought it was a joke so I pulled him down and got some cover. I looked up and he was moving to the right. 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," he said.
Multiple law enforcement agencies, including Simi Valley and Oxnard police, arrived to help sheriff's deputies and SWAT officers. FBI agents also descended on the scene.
Authorities identified the suspected shooter as 28-year-old Ian David Long, a former machine gunner and decorated combat veteran of the U.S. Marines, according to sheriff's officials. They added that they believe Long fatally shot himself after the massacre. He was found inside an office near the entrance of the venue.
The motive for the shooting is unknown.
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Long's gun, which appears to be a .45-caliber handgun, was purchased legally, according to ABC News. AP reports that he also deployed a smoke device.
Long lived in the 800 block of Newbury Park with his mother, about 5 miles from the shooting scene. Deputies responded last April to the home over a call of a family disturbance. Mental health specialists talked to the suspect that day and cleared him, Dean said. Law enforcement officials were at the Newbury Park home as part of the ongoing investigation.
Deceased victims' families devastated
Ventura County sheriff's Sgt. Eric Buschow, as well as Sheriff Geoff Dean, said they did not want to count the suspect among the 12 victims.
In an emotional press conference, a Camarillo man confirmed his son, 22-year-old Cody Coffman, was one of the victims killed in the tragedy.
"This was my first-born son. I am speechless and heartbroken," his father said, as tears ran down his face. "My life has changed forever."
VIDEO: Father identifies 22-year-old son as one of 12 slain in massacre in heartbreaking press conference
"Sister Sister" actress Tamera Mowry-Housley and her husband released a statement confirming that their niece, Alaina Housley, was among those killed.
The couple had been frantically searching for Alaina since the early morning hours in the wake of the deadly shooting.
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Justin Meek, 23, was also confirmed to be among the victims, according to his alma mater California Lutheran University.
Sheriff's sergeant 'died a hero'
During a 3 a.m. press briefing, Dean identified the sergeant as Ron Helus, a 29-year veteran of the department who considered retiring within the next year. At points he choked up, but said Helus "died a hero."
RELATED: Thousand Oaks shooting: Sheriff Sergeant Ron Helus was 'hard working, dedicated' and 'died a hero'
Helus was the first officer inside the door, Dean said.
"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 because he went in to save lives, to save other people," he said.
Helus was immediately hit with multiple gunshots, Dean said. The highway patrolman pulled Helus out, then waited as a SWAT team and scores more officers arrived. Helus died early Thursday at a hospital.
RELATED: Hero saves as many as 30 people during Thousand Oaks mass shooting
By the time officers entered the bar again, the gunfire had stopped, according to the sheriff.
Dean also said in his 41 years in law enforcement, he'd never experienced a mass shooting. He described the scene as "blood everywhere." He later revealed the massacre happened on his last day serving as sheriff.
"I never thought I would see the things around the country that would happen, but I've learned it doesn't matter what community you're in, it doesn't matter how safe your community is, it can happen anywhere," he said.
PHOTOS: Mass shooting at nightclub in Thousand Oaks, California
Helus was transported at 10 a.m. via motorcade from Los Robles Hospital to the Ventura County Medical Examiner's Office in the city of Ventura.
Firefighters honored Helus with a giant U.S. flag raised by two ladder trucks over the route. People lined the streets as the hearse went by.
President Donald Trump tweeted that he was "fully briefed on the terrible shooting." He praised law enforcement, saying "Great bravery shown by police" and said "God bless all of the victims and families of the victims."
....Great bravery shown by police. California Highway Patrol was on scene within 3 minutes, with first officer to enter shot numerous times. That Sheriff’s Sergeant died in the hospital. God bless all of the victims and families of the victims. Thank you to Law Enforcement.— Donald J. Trump (@realDonaldTrump) November 8, 2018
The website for Borderline showed that Wednesday nights are College Country Nights that last from 9 p.m. until 2 a.m. Colleges near the club are California Lutheran University and California State University, Channel Islands, as well as Moorpark, Ventura and Oxnard College.
"Unfortunately our young people, people at nightclubs, have learned that this may happen, and they think about that," the sheriff said. "Fortunately it helped save a lot of lives that they fled the scene so rapidly."
Students from Pepperdine University in Malibu also often attend College Country Nights. The school later confirmed that at least 16 of its students were in the venue at the time of the incident. They said only one of those students was unaccounted for -- until it was learned victim Alaina Housley, who was a freshman at the campus, was killed.
Statement from @pepperdine regarding reported mass shooting at #Borderline in #ThousandOaks. Confirms several #Pepperdine students were there tonight along with students from colleges and community colleges all over the area. Pepp‘s my alma mater. Just awful. @ABC7 pic.twitter.com/ilcQx7WHu1— Greg Lee (@abc7greg) November 8, 2018
Aside from victim Justin Meek, it's unclear how many other students from California Lutheran University were at the bar during the shooting. The campus sent an alert to students that all classes are canceled for the day.
A family unification center was set up at 1375 E. Janss Road, where several devastated families waited to hear news about their loved ones.
An emergency hotline has been established at (805) 465-6650.
Multiple blood drives are being held to help the victims of the shooting. To learn more on how to help those affected, click here.