The suspect may have been suffering from PTSD.
Long was later found dead inside the nightclub.
The bar was filled with people attending "college night" at the Borderline Bar & Grill when Long using smoke bombs and a handgun stormed the venue at approximately 11:20 pm.
Hundreds tried to flee the bar by using barstools against the second-story windows. Among those deceased included sheriff's sergeant, Ron Helus, who was the first to enter the building.
"It's a horrific scene in there," said Ventura County Sheriff Geoff Dean in a conference held in the parking lot. "There's blood everywhere."
Dean continued saying these types of crimes can happen anywhere.
“It doesn’t matter how safe your community is, it doesn’t matter how low your crime rate is — there are people who just don’t think properly everywhere, I don’t care where you are, and they commit horrific acts like this. There’s no way to process,” Dean said. “There’s no way to make sense out of the senseless.”
This tragedy follows a growing trend of mass public violence this year. In February, 17 classmates and teachers were killed in a school shooting in Parkland Florida, and nearly two weeks ago 11 people were shot and killed at a synagogue in Pittsburgh.
In response to the news, President Trump said via Twitter on Thursday that he has been "fully briefed on the terrible shooting. Great bravery shown by police." He added: "God bless all of the victims and families of the victims."