THOUSAND OAKS, Calif. — Thirteen people are dead after a gunman opened fire on a crowd at a country dance bar holding a weekly "college night" Wednesday in Southern California, sending hundreds fleeing in terror including some who used barstools to break windows and escape, authorities said.

The dead included 11 people inside the bar, the gunman 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 said at a news conference early Thursday in the parking lot of the Borderline Bar & Grill. "There's blood everywhere."

The gunman was tall and wearing all black with a hood over his head 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 more minor injuries, including some that came from the 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 waited as a SWAT team and scores more officers arrived.

By the time they entered the bar again the gunfire had stopped, and they found 12 people dead inside, including the gunman.

It's not yet clear how the gunman died, and authorities do not yet know his name or have any idea of a motive, Dean said.

It was college night and country two-step lessons were being offered Wednesday at the Borderline, according to its website.

Pepperdine University said in a tweet that multiple of its students were at the bar. School officials were working to identify and provide support for those students.

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.

Tayler Whitler, 19, said she was inside the bar when a gunman walked in with his face partly covered by something resembling a ski mask, opened fire on a person working the door, then began to shoot people at random.

"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."

The killed sheriff's deputy, 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."

It has been "quite some time" since there was a shooting of any kind in Thousand Oaks, a city of about 130,000 people about 40 miles west of Los Angeles, just across the county line.

Nick Steinwender, student body president at nearby California Lutheran University, rushed to the scene when he heard about a shooting at the bar where he knew friends and fellow students were inside.

"It was chaos, people jumping out of windows, people hopping over gates to get out" Steinwender told KABC.

He said he heard from people inside that they were hiding in bathrooms and the attic of the bar.

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AP journalists Andrew Dalton and Michelle A. Monroe contributed to this report. TEGNA staff also contributed.