Authorities found the gunman dead inside an office adjacent to the bar's entryway, Dean said.
In a cruel twist of fate, dozens of people who survived the Las Vegas shooting were inside the Borderline bar when Long opened fire.
Dean described the scene inside the bar as "like hell".
The Marine Corps said Long earned several awards, including a Combat Action Ribbon and a Marine Corps Good Conduct Medal.
"I'm super upset that it happened in our home, and I feel terrible for the families that have to go through this", Merrill said at the vigil.
A witness who was inside the bar during the shooting said she heard "very, very loud gunshots".
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.
The sheriff's department had several previous contacts with Long including a call to his home in April when deputies found him acting irate and irrationally.
He saw a California Highway Patrol officer who had just pulled over a driver and shouted to him that there was shooter inside the building. Helus had served Ventura County for 29 years and was planning to retire in a year or so, according to Ventura County Sheriff Geoff Dean.
Knapp said, "It took a couple of seconds for people to realize what was going on and once that happened it was just utter chaos". "And he didn't come last night", she told the Times. Can you do that for me?
Mr Matt Wennerstron, a 20-year-old college student and regular at the bar, said the shooter fired a short-barrelled pistol that apparently had a 10 to 15-round magazine.
He said he and others smashed their way out of the bar onto a balcony and then jumped down to safety.
Gunman Long (left and right) was a machine gunner in the Marines Corps and served until 2013 when he was given honorable discharge.
Map of California locating shooting at a bar in Thousand Oaks. Eleven people inside the bar were killed along with a cop who was one of the first on the scene.
TV footage showed SWAT teams surrounding the bar, with distraught revellers milling around and using their cell phones as lights from police cars flashed.
It also marks the deadliest mass shooting in the country since 17 people were killed earlier this year at a high school in Parkland, Florida.
The mother of a young man who survived last year's gun massacre in Las Vegas only to be killed by another gunman in a California bar this week has made an impassioned plea for gun control. "I want those b******* in Congress - they need to pass gun control so no one else has a child that doesn't come home".
Trump touted his efforts to fund PTSD among veterans.