A bomb-disposal unit checks around the apartment building where Omar Mateen is believed to have lived in Fort Pierce, Fla. (Joe Raedle/Getty Images)

FORT PIERCE, Fla. — At a high school in Florida, students watched the horrors of Sept. 11, 2001, unfold on live TV. When the second hijacked airliner slammed into the World Trade Center’s south tower, the class sat in stunned disbelief. But one student, a classmate recalled, “started jumping up-and-down cheering on the terrorist.”

That was sophomore Omar Mateen, according to one of the accounts from former students in Stuart, Fla., remembering 9/11 and the reaction by the student who, nearly 15 years later, would carry out the worst mass shooting in U.S. history.

The recollections of Mateen’s actions could not be independently verified, and the memories could be clouded by the years that have passed. But similar versions were detailed in separate interviews. As the snapshot in time, the recollections appear to offer yet another stitch in the wider tapestry of Mateen’s life and views before Sunday’s rampage, which included his pledge of loyalty to the Islamic State during a call to police during the standoff.

In an interview, Robert Zirkle, then a freshman at Martin County High School, said he saw Mateen excited and making fun of how America was being attacked on 9/11. “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.”

“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,” he added, noting that Mateen attended the Spectrum Alternative School, a separate campus in Stuart for students with poor grades or behavioral issues.

“After 9/11 happened, he started changing and acting different,” Zirkle said.

Hours after the carnage on Sunday in Orlando, several former classmates of Mateen began talking in a group chat on Facebook.

The gunman who killed at least 49 people in a shooting rampage at an Orlando nightclub has been identified as 29-year-old Omar Mateen. Here is what we know about him so far. (Deirdra O'Regan/The Washington Post)

One former student told Zirkle on Facebook that on 9/11, the students were watching the TV in class. On the group chat, the former student wrote that Mateen “stood up in class during the 9/11 attack and after the second plane hit the building he started jumping up-and-down cheering on the terrorist.”

One former student who was sitting in the same class as Mateen remembers the morning of 9/11 clearly: “Teachers said turn on the TV. We see the one plane hit. And then see second plane hit … [Mateen] was smiling. It was almost like surreal how happy he was about what happened to us.”

The former student said Mateen went on to claim that Osama bin Laden was his uncle. The ex-classmate spoke on the condition of anonymity for fear of business clients finding out he attended an alternative school. Mateen’s claim of family links to bin Laden was also mentioned by Zirkle.

In a separate phone interview, another former classmate said he remembered Mateen acting out in class when the towers were hit because both of them were sent to the dean’s office at the same time for misbehaving. The former student spoke on the condition of anonymity because he feared being overwhelmed by media requests.

“I was sleeping in class and woke up to see people jumping off buildings, so I started swearing and they sent me up,” the former student said. “But Omar was saying some really rude stuff. Stuff like, ‘That’s what America deserves.’ That kind of thing. It wasn’t right.”

Both that former student and Zirkle recalled Mateen being suspended or expelled from the school shortly after 9/11. The other former classmate — who shielded his identity because of business clients — vividly recalled Mateen’s father picking him up after school on 9/11.

“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.”

“He got bullied a lot,” said the former student who sat in the dean’s office with Mateen. “It may have been because he was Muslim. But high school can be rough; people can pick on you just because of your name.”

“They had to escort him out of the school,” Zirkle said. “Other kids were trying to fight him. A couple days after, they had to take him off the bus.”

“A few of my friends wanted to fight him because he kept doing it and saying crazy things,” he added. “It’s weird. He was totally cool before 9/11, and then something changed.”