A hooded gunman wearing all black opened fire at a country dance bar holding a weekly “college night” in Thousand Oaks, using a handgun and a smoke device to kill 12 people and sending hundreds fleeing, authorities said Thursday.

Terrified revelers used barstools to break second-floor windows and jump to safety to escape the dance bar, where the gunman was later found dead. Those killed in the shooting Wednesday night also included 11 people inside the bar and a sheriff’s sergeant who was the first officer inside the door, Ventura County Sheriff Geoff Dean said.

“It’s a horrific scene in there,” Dean told a news conference in the parking lot of the Borderline Bar & Grill. “There’s blood everywhere.”

In a news conference about 7 a.m., Ventura County Sheriff Geoff Dean identified the alleged shooter as Ian David Long, 28, who lived in the area, and he said Long was armed with a single Glock handgun that had an “extended magazine” that would allow it to hold more than the standard number of 10 bullets that a magazine would hold.

Long, a U.S. Marine Corps veteran, is believed to have killed himself, Dean said, adding that a motive for the crime was not known. Law enforcement officials from several agencies, including the FBI, were going through the house in the Ventura County community of Newbury Park, where Long lived with his mother.

Long Beach police said on Twitter this morning that the department will step up patrols of local entertainment districts in the wake of the shooting.

We are deeply saddened by reports of the tragic shooting in Ventura. Our thoughts & prayers go out to the victims and their families. @LBPD will maintain an increased presence & dedicated patrols in our entertainment districts and will continue to monitor events as they unfold. — Long Beach PD (CA) (@LBPD) November 8, 2018

The massacre was the deadliest mass shooting in the United States since 17 classmates and teachers were gunned down at a Parkland, Florida school nine months ago. It also came less than two weeks after a gunman killed 11 people at a synagogue in Pittsburgh. That, it turn, closely followed the series of pipe bombs mailed to prominent Democrats, CNN and former officials critical of President Donald Trump.

Trump said Thursday on Twitter that he has been “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.”

The gunman at the dance bar was tall and wearing all black with a hood and his face partly covered, witnesses told TV stations at the scene. He first fired on a person working the door, then appeared to open fire at random at the people inside, they said.

Many more people had minor injuries, including some that came from their attempt to flee, Dean said.

Sheriff’s Sgt. Ron Helus and a passing highway patrolman were responding to several 911 calls when they arrived at the Borderline Bar & Grill in Thousand Oaks at about 11:20 p.m., the sheriff said. They heard gunfire and went inside.

Helus was immediately hit with multiple gunshots, Dean said. The highway patrolman cleared the perimeter and pulled Helus out, and then waited as a SWAT team and scores more officers arrived. Helus died early Thursday at a hospital.

By the time they entered the bar again the gunfire had stopped. They found 12 people dead inside, including the gunman. It’s not yet clear how the gunman died, Dean said.

The shooting happened on college night. Two-step lessons in country dancing were being offered Wednesday at the Borderline, according to its website.

The bar, which includes a large dance hall with a stage and a pool room along with several smaller areas for eating and drinking, is a popular hangout for students from nearby California Lutheran University who enjoy country music. It’s also close to several other universities including California State University Channel Islands in Camarillo, Pepperdine University in Malibu and Moorpark College in Moorpark.

When the gunman entered, people screamed and fled to all corners of the bar, while a few people threw barstools through the windows and helped dozens to escape, witnesses said.

Video from the scene accessed by The Associated Press is punctuated by loud sounds of several rounds of gunfire. A terrified witness runs out. Police cars are seen arriving and an armed officer takes up position outside the bar. Three men rush out carrying a bloodied fourth individual. They try to stem the bleeding of what appears to be a gunshot wound.

Cole Knapp, a freshman at Moorpark College, said he was inside the bar when the shooting began but thought at first that it was “just someone with an M-80, just kind of playing a prank.”

Then he said he saw the shooter, wearing a black beanie and black hoodie and holding a small caliber handgun.

“I tried to get as many people to cover as I could,” Knapp said. “There was an exit right next to me, so I went through that. That exit leads to a patio where people smoke. People out there didn’t really know what was going on. There’s a fence right there so I said, ‘Everyone get over the fence as quickly as you can, and I followed them over.”

He said a highway patrol officer was nearby who just happened to be pulling someone over.

“I screamed to him, ‘There’s a shooter in there!’ He was kind of in disbelief, then saw that I was serious.”

Knapp said he has friends who haven’t been accounted for.

Tayler Whitler, 19, said she was on the dance floor with her friends nearby when she saw the gunman shooting and heard screams to “get down.”

“It was really, really, really shocking,” Whitler told KABC-TV as she stood with her father in the Borderline parking lot. “It looked like he knew what he was doing.”

Sarah Rose DeSon told ABC’s “Good Morning America” that she saw the shooter draw his gun.

“I dropped to the floor,” she said. “A friend yelled ‘Everybody down!’ We were hiding behind tables trying to keep ourselves covered.”

Shootings of any kind are very rare in Thousand Oaks, a city of about 130,000 people about 40 miles (64 kilometers) west of Los Angeles, just across the county line.

Helus was a 29-year veteran of the force with a wife and son and planned to retire in the coming year, said the sheriff, who choked back tears several times as he talked about the sergeant who was also his longtime friend.

“Ron was a hardworking, dedicated sheriff’s sergeant who was totally committed,” Dean said, “and tonight, as I told his wife, he died a hero because he went in to save lives.”

AP journalists Andrew Dalton in Los Angeles, Michelle A. Monroe in Phoenix and Michael Balsamo in Washington contributed to this report.