They went through windows.
Thousand Oaks is in Ventura County, about 40 miles from downtown Los Angeles.
He attained the rank of corporal and saw combat while deployed in Afghanistan from November 2010 to June 2011.
Ms Teylor Whittler managed to escape the carnage unhurt. "He looked like he knew what he was doing".
"It's a horrific scene in there", Mr Dean said in a press conference at a vehicle park.
The mass shooting happened at Borderline Bar & Grill in Thousand Oaks.
"He went in there to save people and paid the ultimate price", he said.
Of the 12 victims was Ventura County Sheriff's Sgt. Ron Helus, who immediately charged inside to confront the gunman. "I love you. I'll talk to you later,"' Dean said. California Highway Patrol was on scene within 3 minutes, with first officer to enter shot numerous times.
Mr Helus, a sergeant with the Ventura County Sheriff's Office, was talking to his wife when calls started coming in about a shooting at the Borderline Bar & Grill.
Knapp led others to an outdoor patio and over a fence to safety.
Another friend said he was part of a "community there" who would frequent the bar.
He first fired on a person working the door, then appeared to shoot at random at people inside, they said.
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Dean described the scene inside the bar as "like hell".
Sheriff Dean also revealed Long was known to the department following a number of "minor events" in recent years.
The gunman used a Glock 21.45-caliber handgun created to hold 10 rounds plus one in the chamber, but the gun had an extended magazine, the sheriff said.
Map of California locating shooting at a bar in Thousand Oaks.
At least a dozen people were killed - including a "hero" cop who was due to retire next year and died bravely trying to save others' lives.
"It's a gun culture", he said.
TV footage showed SWAT teams surrounding the bar, with distraught revelers milling around and using their cell phones as lights from police cars flashed.
Authorities identified a Facebook post believed to have been made by the shooter around the time of the attack, according to a law enforcement official familiar with the ongoing investigation. A sheriff's deputy, 12 attendees and the gunmen were killed. And he didn't come home last night, and the two words I want you to write are: 'gun control.' Right now, so that no one else goes through this.
At the White House, Trump touted his efforts to fund work on PTSD among veterans.
Susan Schmidt-Orfanos, whose son, Telemachus Orfanos, was killed Wednesday night after surviving a mass shooting previous year in Las Vegas, said she wanted gun control - not platitudes.
A survivor of the Las Vegas shooting massacre a year ago was killed in Wednesday's shooting at a popular Southern California country bar.