Authorities haven't determined a motive.
The official - who was briefed on the investigation but not authorized to discuss it publicly and spoke to The Associated Press on condition of anonymity - would not give additional details on what the 28-year-old former Marine posted on his Facebook and Instagram accounts.
"I don't know what he saw over there, but he was mentally disturbed before he left for the military", Dominique Colell, who coached Long in track, told Inside Edition.
President Donald Trump has described the gunman responsible for killing 12 people in a California bar as "a very sick puppy". Dean told the Los Angeles Times that the amount of blood within the vicinity made it hard to determine whether Long killed himself or was shot to death by police.
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Apart from 11 people in the bar, the suspect shot and killed sheriff's sergeant Ron Helus, a 29-year veteran of the department who considered retiring within the next year. "They talked to him", Ventura County Sheriff Geoff Dean said.
"My son was in Las Vegas with a lot of his friends and he come home", she told ABC's Los Angeles affiliate.
Typically after incidents like this week's shooting, regular Americans, politicians, and celebrities offer their "thoughts and prayers" to the victims, which has turned the phrase into a meme about inaction on gun control. "The last thing I said was, 'Son, I love you". "I want those b******* in Congress - they need to pass gun control so no one else has a child that doesn't come home".
A gunman shooting from the 32nd floor of a hotel and casino with high power weapons killed 58 people. She was appalled that such bloodshed had come to her community. "I already didn't wish it on anybody to begin with for the first time", Kelly said outside his home in Thousand Oaks.
At the White House, Mr Trump touted his efforts to fund work on PTSD among veterans. He declined to engage on questions on whether the nation needs stricter gun control laws.
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Long grew up in Thousand Oaks and several people who knew him described him in disturbing terms.
"I literally feared for myself around him", Colell said in an interview Friday.
Police said Long used a.45 caliber Glock handgun with an extended magazine to storm the Thousand Oaks bar, where patrons as young as 18 were enjoying "college country night".
About 18 months ago, Don and Effie MacLeod heard "an bad argument" and what he believes was a gunshot from the Longs' property.
And tonight, as I told his wife, he died a hero because he went in to save lives.
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