He said the gunman had apparently shot himself.
The Borderline Bar shooting left 12 dead, including the police officer who responded to first reports.
Dean said the suspect had an extended magazine for his gun, and it was not clear whether he reloaded during the attack. "Right now as far as we know there was only one handgun, but that could certainly change as we do a more thorough search of the building".
He and the CHP officer went inside, according to Ventura County Sheriff Geoff Dean, who held an emotional news conference outside the bar early Thursday morning.
Long was a veteran and served in the U.S. Marine Corps.
Numerous patrons were local students from nearby California Lutheran University, ages 19 to 25, attending "College Country Night".
After the shooting was over, Kelly said he and another Marine friend helped victims alongside first responders.
"He went in there to save lives".
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He added that it had "been a privilege" to work under Sessions. "He is a man of integrity who has served this nation well". If they had a secular worldview, he said that would make him "very concerned about how they judge".
"We do not know how many rounds were in the weapon", he said. A highway patrolman pulled him out and waited for scores of other officers to converge on the bar.
"He died tonight going in to try to save others and paid the ultimate sacrifice", Dean said.
Holden Harrah, who was at the bar, told CNN the gunman walked in and opened fire within seconds.
There were hundreds of people packed inside Borderline Bar and Grill when a gunman burst inside throwing smoke grenades and firing dozens of rounds, witnesses and authorities said.
Nick Steinwender, Cal Lutheran student body president, told KTLA-TV he immediately started receiving messages about the shooting, and he and his roommate went to the scene to offer rides back to campus or moral support. "But I've learned, it doesn't matter what community you're in, it doesn't matter how safe your community is". Choking back tears, Jason said, "only he and I know how much I love him, how much I miss him".
Another witness of the California shooting, Brendan Kelly, also said he was at the Las Vegas massacre.
"There were people hiding in restrooms, people hiding in attics", he added.
"There are a few people we can't get a hold of, but in these situations, people usually turn off their phones to be safe so I'm not gonna get too anxious.
It's something that I wish I was OK to do, but unfortunately I can't, so that's why I'm making sure that he gets remembered", King said. "What I saw and experienced at Route 91 reaffirmed that I want to be a firefighter", he said. He was identified as Sheriff's Sgt. Ron Helus, a 29-year veteran about a year from retiring.