As a teenage high school student in Florida, gay nightclub mass murderer Omar Mateen cheered as terrorists flew jumbo jets into the Twin Towers on 9/11, his former classmates recalled.

Self-proclaimed Islamic State devotee Mateen was a sophomore when the al Qaeda terrorists attacked New York on Sept. 11, 2001 — and classmates vividly recalled his offensive reaction to America’s darkest day, according to the Washington Post.

One former classmate said he was in the same room with Mateen during the 9/11 attacks. While everyone in the classroom was shocked or angered, Mateen was celebrating.

“But Omar was saying some really rude stuff. Stuff like, ‘That’s what America deserves.’ That kind of thing. It wasn’t right,” the former classmate said.

That student also remembers the school sending Mateen home — and the killer’s dad wasn’t pleased about having to pick him up.

“I remember his dad walking up,” the former student said. “And in the courtyard in front of everyone, the kid’s dad slapped him right across the face.”

Here’s why Mateen was able to get a gun, despite all of the red flags:

Robert Zirkle, who was a year behind Mateen, said the man who would commit the second-most deadly act of terror on US soil, killing 49 and wounding 53 others, expressed glee over 9/11 from that point forward.

“He was making plane noises on the bus, acting like he was running into a building,” Zirkle recalled.

“I don’t really know if he was doing it because he was being taught some of that stuff at home or just doing it for attention because he didn’t have a lot of friends.”

The events of 9/11 seemingly changed Mateen from the normal, shy teen he once was.

“Before 9/11 happened, we were pretty straight. We all rode the same bus. We weren’t really close friends, but friends at least a little,” Zirkle said. “After 9/11 happened, he started changing and acting different.”