As he carried out the shooting, he posted messages to his Instagram story in real time, the site reported.
Those videos remained up on Long's social media profile for hours after the shooting, before being discovered by police who asked Instagram to remove his account, according to the same report.
Some of the messages may have indicated the shooting was the result of a grudge, it said.
Police continue to search for a motive or any clear understanding of why Long walked into the Borderline Bar and Grill late on Thursday and killed as many people as he could.
Videos from inside the venue showed a horrific scene as people fled the attack.
Some of those injured and killed inside the country music venue had survived the festival in Las Vegas where Stephen Paddock opened fire and killed 58 people in the most deadly mass shooting in US history.