Hundreds of people were inside the Borderline Bar & Grill in Thousand Oaks when Long, armed with an uzi, unleashed a terrifying barrage of smoke bombs before opening fire and killing 12 people, leaving "blood everywhere".
The killer was identified as Ian David Long, a 28-year-old veteran who authorities said had an episode of erratic behavior last spring that was thought to be post-traumatic stress disorder because of his military background.
In a news conference, Dean said 12 victims were killed, including Ron Helus, a sergeant from his department who was one of the first responders to the scene.
Civilians and other deputies pulled Sergeant Helus from the bar and he was taken to Los Robles hospital, where he died an hour later. Some men ran toward them and said the gunman was coming.
"He died tonight going in to try to save others and paid the ultimate sacrifice", Dean said.
He could not immediately provide information on the extent of the victims' injuries.
"With over 2,500 square feet of wide open dance floor, Borderline has provided a haven for country line dancing folks of all ages equipped with pool tables", the website says.
Long was a veteran and served in the U.S. Marine Corps. It was not clear whether he killed himself or was killed by officers, Dean said.
Sheriff's deputy speaks to a potential witness as they stand near the scene, November 8, 2018, in Thousand Oaks, Calif. where a gunman opened fire Wednesday inside a country dance bar crowded with hundreds of people.
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Officials said that shots were still being fired when the first officers arrived at the scene.
Sgt. Helus had a wife and son, and was due to retire in the next year from Ventura County, California Sheriff's Office. "As far as I know, all of my friends are OK, safe", he said.
The bar is located in Thousand Oaks, Ventura County, about 40 miles northwest of Los Angeles.
The gunman opened fire at around 11:20 p.m. (1:20 a.m. CT Thursday).
But most agree that there was a shooting and many people fled the bar and grill.
"Within a split second, everybody yelled "get down", she said.
"It's been a rough night for all of us", Buschow said, according to the report.
Madeleine Carr, the paper's editor, told CNN on Thursday morning that she was also waiting to hear back from some friends who had gone to the bar to attend College Country night.
There was no screaming, she said.