At a high school in Florida, students watched the horrors of September 11, 2001, unfold on live TV. When the second hijacked airliner slammed into the World Trade Center's south tower, the class watched 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 Martin County High School remembering 9/11 and the reaction by the student who, nearly 15 years later, would carry out the worst mass shooting in US 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 they 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 the Martin County 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. "After 9/11 happened, he started changing and acting different."

Hours after the carnage on Sunday in Orlando, several classmates from Mateen's school days at Martin County High School and Spectrum Alternative School began talking in a group chat on Facebook.

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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."

In a phone interview, a third classmate who did not want to be identified for fear of being bombarded by news media, 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.

"I was sleeping in class and woke up to see people jumping off buildings, so I started swearing and they sent me up," the 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 student and Zirkle recalled Mateen being suspended or expelled from the school shortly after 9/11.

"He got bullied a lot," said the 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 a totally cool before 9/11, and then something changed."

In another later incident, Zirkle said Mateen told some of his friends in class that Osama bin Laden was his uncle, which started another fight. "They had to escort him out of the school then as well."