Sheriff Geoff Dean said police had "no idea" what the motive was for the shooting, which took place in at Borderline Bar and Grill during a crowded "college night" event.
"I've learned it doesn't matter what community you're in", Dean said.
Paul Delacourt, assistant director of the FBI's Los Angeles office, said agents are processing Long's home and vehicle in an effort to identify his motivation and paint a picture of his frame of mind. Dean says Helus calls his wife several times every shift, according to ABC News. He said he thought their injuries were minor, and that majority were likely injured as they escaped, some by breaking windows.
Long was "the victim of a battery" at a bar in Thousand Oaks in January 2015, Dean said. But a mental health professional who spoke with Long said he did not believe Long needed to be hospitalized.
Long served in the Marine Corps from August 2008 to March 2013, with a deployment in Afghanistan between November 2010 and June 2011, military records show.
Ventura County Sheriff Geoff Dean said on Thursday that Long was armed with a Glock 21, a.45-caliber handgun created to hold 10 rounds plus one in the chamber.
President Donald Trump, who has resisted a surge in calls for tougher gun controls since 17 students were shot dead at Stoneman Douglas High School in Florida earlier this year, said on Twitter that he had been "fully briefed on the awful shooting in California". It's unclear whether the magazine was legally purchased. "We don't know if he reloaded his weapon or not, we're still interviewing witnesses".
Berge said when he saw police activity at the house on Thursday morning, "I knew what it was".
"It appears he walked up to the scene".
Long shot an unarmed security guard outside the bar before going inside, where he fired on security staff, CNN reported. The suspect reportedly threw smoke bombs inside the bar.
Helus and a California Highway Patrol officer, who hasn't been publicly named, were on scene within minutes, and were the first responders to fearless their way into the bar, darkened by a smoke bomb set off by Long.
Sgt. Ron Helus, among the first law enforcement officers to respond, died at a hospital hours later from injuries sustained in a confrontation with the shooter.
"It's a horrific scene in there", Dean told a news conference in the parking lot of the Borderline Bar & Grill.
In a joint statement, the couple said: "Alaina was an incredible young woman with so much life ahead of her and we are devastated that her life was cut short in this manner". "I'll talk to you later".
"I tried to get as many people to cover as I could", Knapp told the Associated Press. "And the gunman started opening fire on the cashiers, the people working the cash register on the front desk".
President Donald Trump tweeted that he had "been fully briefed on the bad shooting", the latest chapter in America's epidemic of gun violence that came just 10 days after a gunman killed 11 worshippers at a synagogue in Pittsburgh.
Multiple people have been injured, including a deputy, in a shooting at a bar in Thousand Oaks.
"Hey, I gotta go handle a call. You know, I think it's going to be something that we're going to have to come together and move past".
Thousand Oaks, with a population of about 127,000, is a leafy, sprawling suburb named the third safest city in the United States for 2018 by the Niche research company.
The Sheriff's Office set up a hotline for those who may be affected by the shooting, at 805-465-6650.