According to survivors, Long was dressed in all-black, wearing a baseball cap, sunglasses and a mask covering the bottom part of his face.
Investigators have not commented on a motive for the rampage or whether mental illness played a role.
"I looked him in his eyes while he killed my friends I hope he rots in hell for eternity", he wrote on Instagram.
The sheriff said Long drove to the bar and shot an unarmed security guard who was standing outside before opening fire at random inside.
He died at a hospital early Thursday.
Ventura County Sheriff's Office Capt. Garo Kuredjian said the first reports of shots fired came around 11:20 p.m.at the Borderline Bar & Grill in Thousand Oaks, which is about 64 kilometers west of Los Angeles.
Clockwise from left: Telemachus Orfanos served with the USA navy for two years.
'I don't want prayers.
His mother, Susan Orfanos, spoke about the tragedy to The New York Times, and she told the publication, "He was killed last night at Borderline". On his way to the bar, Helus was talking on the phone with his wife, his last words to her as obtained by the Los Angeles Times were "Hon, I got to go, I love you".
Speaking of how Orfanos had survived the Las Vegas massacre, where 58 people were shot dead, his father said that the 27-year-old lost his life at his hometown.
"He survived the Vegas massacre a year ago where he spent the whole night carrying injured and assisting first responders", Krisilas told USA Today. "But later I smelled the gun fire, then I knew it's real", she said.
Jerry Nadler, the top Democrat on the House Judiciary Committee, said that lawmakers would get to work on legislation including universal background checks when the House of Representatives convenes in January with a Democratic majority. But he declined to engage on questions on whether the nation needed stricter gun control laws.
"It feels like a dream, and I wish that I can just wake up from it and hear from both of them again", Piasecki said.
Thousand Oaks, ranked the third safest city in the United States in 2018 by website Niche, was added to the list of victims of gun violence in a country divided over its gun policies.
About 18 months ago, Don and Effie MacLeod heard "an bad argument" and what he believes was a gunshot from the Longs' property.
Deputies were called to Long's house in response to a disturbance in April this year and "felt he might be suffering from PTSD, the fact he was a veteran and had been in the corps", he said.
"All of a sudden we heard the "bang, bang" of the gunshots and it just started going insane and people were pushing".
"He was just a great guy, a gentle soul", Buschow said. "Tonight Thousand Oaks takes its place with those cities, who in order to move forward will rely on hope".
An evacuation site was at capacity at the Thousand Oaks Teen Center, which had also served as a family assistance and reunification site following the shooting.