"It's a horrific incident", Dean told reporters. They say the gunman did not say anything. The official said the 29-year-old man deployed a smoke device and used a.45-calibre handgun.
Authorities urged the public to avoid the area.
Here's what we know about the late Wednesday shooting. "That's never going to happen - all because of the stupid choice that killer made that robbed so many people of hearing Justin Meek's attractive voice".
Once again, President Donald Trump has ordered flags at half-staff "in a mark of solemn respect".
Dean said Long had previous run-ins with police, including one instance in April where he acted "a little irrationally".
The gunman was tall and wearing all black with a hood and his face partly covered, witnesses told TV stations.
"They found 11 victims that had been killed", said Dean of the first response unit before detailing that the death of Helus brought the toll to 12, not including the gunman.
His Facebook profile is full of photos of the officer and his son hunting, fishing, hill-walking and playing with their dog together. The highway patrolman pulled Helus out, then waited as a SWAT team and scores more officers arrived.
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He called his wife shortly before going into the nightclub.
ABC Reporter Marc Brown said: "We are hearing that some of the people who were at Borderline, were also at the Route 91 Harvest Festival in Las Vegas, scene of the worst mass shooting in modern USA history".
Dean said the other prior encounters were a traffic accident and an incident when he was the victim of battery at a bar.
One of whom also survived the Vegas shooting, the LA Times reported.
Hundreds of people were attending a college night at the Borderline Bar & Grill - about 65 kilometres northwest of Los Angeles - when the gunman opened fire Wednesday night. The venue, which describes itself as the county's largest country dance hall and live music venue, was hosting a "College Country Night".
Nick Steinwender, who is part of the university's student government, said his friends were inside the bar when the shooting started. Last thing I said was, "son, I love you".
When the gunman entered, people screamed and fled to all corners of the bar, and a few threw barstools through the windows and helped dozens to escape, witnesses said. Law enforcement and emergency crews were flooding the scene.
"We don't know who shot anybody at this point". The sheriff believes Dean took his own life.
He said the only weapon found in an initial sweep was a handgun.
"People started running to the back door", she said. "I just wanted to get down as soon as I heard the first shot go off". Champion's love of country music drew him to both tragic events, and he said that fans are "all a big family, and unfortunately this family got hit twice".