|This undated file image shows Omar Mateen, who authorities say killed dozens of people inside the Pulse nightclub in Orlando on Sunday, June 12, 2016.|
9mm handgun that also was recovered when gunman was ultimately killed by police.
The existence of the video was first reported by The Washington Post.
The disclosures offer an increasingly chilling account of the massacre, building on the release earlier this week of partial transcripts of Mateen's telephone contacts with police dispatchers and crisis negotiators while holed up in a bathroom after the initial attack.
"I am in Orlando, and I did the shooting,'' Mateen said during one of three contacts with a 911 dispatcher. Mateen went on to declare his allegiance to the Islamic State terror group and its leader Abu Bakr al-Baghdadi during the 50-second call.
A summary of the gunman's separate communications with crisis negotiators indicated that Mateen was asked what he had done inside the club, prompting a warning to police who were massing outside.
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"No, you already know what I did,'' he said. "There is some vehicle outside that has some bombs, just to let you know. You people are gonna get it, and I'm gonna ignite it if they try to do anything stupid.''
No explosives were ever found in the gunman's vehicle or in the club.
Federal investigators are still attempting to determine what motivated Mateen to target the club.
Witnesses have suggested that Mateen may have been angered after being snubbed by club patrons. But the official said many of those witness claims cannot be corroborated and that no defining motive has yet emerged.
The gunman's wife has allegedly told authorities that she feared that Mateen was eying the club as a possible target for an attack and had tried to dissuade him. Noor Salman allegedly told authorities of her increasing concerns when Mateen left their Fort Pierce, Fla., home hours before the attack.
He allegedly told his wife that he was going out to meet friends, federal officials have said.