Kevin Johnson

USA TODAY

In the midst of the brutal assault inside Orlando’s Pulse nightclub, security cameras captured Omar Mateen, a powerful assault rifle perched at hip level, firing indiscriminately into crowds of patrons, a federal law enforcement official said Thursday.

The official, who has viewed a compilation of video clips, said some victims can be seen falling to the floor, while other patrons appear frozen or possibly confused by the frenzy unfolding around them.

At some points, the 29-year-old gunman also can be seen firing at victims who have already fallen, said the official, who is not authorized to comment publicly.

It isn't immediately clear from the footage how many times Mateen stopped to reload the .223-caliber Sig Sauer MCX rifle, but the official said the scope of the carnage likely would have required the gunman to reload multiple times during the June 12 attack that left 49 dead and 53 others wounded.

The video reviewed by the official showed Mateen firing only the rifle and not the 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.

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"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.

"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.