A hooded gunman dressed entirely in black has opened fire on a crowd in a Southern California bar, killing 12 people and sending hundreds fleeing, before taking his own life.
The first reports of shots fired came around 11.20pm local time on Wednesday at the Borderline Bar & Grill in Thousand Oaks, which is about 64km west of Los Angeles.
The dead from the shooting were 11 people inside the bar and sheriff's Sergeant Ron Helus who was the first officer inside the door, Ventura County Sheriff Geoff Dean said.
Dean said around 10 other people were shot and wounded.
The country dance bar was holding a weekly 'college night'. The shooting sent hundreds fleeing, including some who used bar stools to break windows and escape, authorities said.
'It's a horrific scene in there,' Dean told a news conference in the parking lot of the bar. 'There's blood everywhere.'
The gunman at the country dance bar was 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 their attempt to flee, Dean said.
Helus and a passing highway patrolman were responding to several 911 calls when they arrived at the bar about 11.20pm, 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, and authorities do not yet know his name or have any idea of a motive, Dean said.
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. 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 bar stools through the windows and helped dozens to escape, witnesses said.
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'.
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.'
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.
The massacre was the deadliest mass shooting in the US 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.