Twelve people, including a police officer, have died after a gunman opened fire at the Borderline Bar & Grill, a country bar in Thousand Oaks, Calif., on Wednesday night (Nov. 7). The venue was hosting "college country night," and some of those in attendance were also at the Route 91 Harvest Festival shooting in October 2017.

CNN reports that several hundred people were at the Borderline Bar & Grill when the shooting -- which Ventura County Sheriff Geoff Dean calls a "tragic, tragic situation" -- began around 11:20PM local time. California Highway Patrol, a SWAT team, the FBI and other officers arrived soon after.

"They found people hiding in restrooms, people hiding in attics," Dean says, noting that, in addition to the deceased, there were “multiple other victims of different levels of injuries.” As of 10AM ET on Thursday (Nov. 8), CNN reported, 21 people had been treated for injures sustained during the shooting and released; an additional 11 patients with minor injuries, all of whom went to local hospitals on their own, had been released as of around 2PM ET.

"I saw the shots go off, although I didn’t really see the guy," says Kayla Simmons, who was in the Borderline Bar & Grill when the shooting began. "I knew something was wrong straight away. I just wanted to get down as soon as I heard the first shot go off."

Nicholas Champion tells CBS that he and "probably 50 or 60 others who were in the building at the same time as me tonight" were also at the 2017 Route 91 Harvest Festival in Las Vegas, Nev., at which 58 people died when a gunman opened fire on the crowd from the nearby Mandalay Bay hotel.

Photos From Outside the Thousand Oaks Shooting

Authorities report that the shooter was 28-year-old U.S. Marine Corps veteran Ian David Long of Newbury Park, Calif., who is dead (he is not counted among the number of deceased). He was killed by a self-inflicted gunshot wound, according to Dean, and was found by the bar's entrance. Long did not have an ID on him at the time of the shooting, but authorities have had "several contacts" with him "over the years," according to Dean.

"[There were] minor events, a traffic collision. He was a victim of a battery at a local bar in 2015," Dean explains. "In April of this year, deputies were called to his house for a subject disturbing. They went to the house, they talked to him. He was somewhat irate. Acting a little irrationally. They called out our crisis intervention team, our mental health specialists, who met with him, talked to him and cleared him. Didn't feel he was qualified to be taken under 5150. And he was left at that scene last April."

Long, USA Today reports, was a tall, white male, and was dressed in black; ABC reports that he went by his middle name, and a neighbor tells the network that he suffered from PTSD and lived with his mother. An anonymous law enforcement source tells the Los Angeles Times that Long was armed with a Glock .45 handgun -- ABC reports that it was purchased legally -- and a "smoke device;" the Associated Press reports that the gun had an extended magazine, which is illegal in the state of California.

Authorities are working to determine if the shooting was terrorism-linked, but as of 10:30AM ET on Thursday, there is no known motive. FBI Assistant Director Paul Delacourt reports, however, that there is no indication that Long acted in conjunction with others, and that there is no other threat the the area.

Dean reports that Long shot a security guard outside of the Borderline Bar & Grill before going inside and shooting other security guards and employees, then opening fire on the crowd. "We don't have any other details to confirm an exact chronology about what happened," he says. "We'll provide that to you as we bring all our witness statements together and we feel more comfortable about talking about that."

The 12 victims are: Cody Coffman; Dan Manrique; Mark Meza; Justin Meek; Blake Dingman; Sean Adler; Noel Sparks; Jake Dunham; Telemachus Orfanos; Kristina Morisette; Alaina Housley; and Sgt. Ron Helus. Helus and a California Highway Patrol colleague were the first to arrive on the scene on Wednesday night. When he entered the venue, Helus "was struck multiple times with gunfire," according to Dean. His partner brought Helus to safety, but the 29-year police veteran later died at the hospital.

"He was hard-working, he was totally committed, he gave his all and, tonight, as I told his wife, he died a hero," says Dean, who noted that Helus leaves behind a wife and child, and that he was looking forward to retiring soon. "He gave his life to save other people."

Country Stars Offer Condolences Following Borderline Bar & Grill Shooting

According to its website, the Borderline Bar & Grill is "Ventura County's Largest Country Dance Hall & Live Music Venue." Borderline has been open since the late 1980s, when it was located in Malibu, Calif.; it moved to Thousand Oaks in 1993. Artists such as Lee Brice, Tyler Farr and more have performed at Borderline over the years. Up-and-comer Aubrie Sellers had planned to go line dancing at the club on Wednesday night, but she stayed home because her cat was sick.

Thousand Oaks is located about 40 miles west of Los Angeles, and was recently ranked as the third-safest city in the United States by Niche.com, which analyzes "crime rates for murder, assault, rape, burglary and other crime statistics by city ... based on data from the FBI and U.S. Census."

President Donald Trump has ordered that flags be flown at half-mast in the U.S. in honor of the shooting's victims.

This story is developing. The Boot will update as further details are made available.

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