Mateen was killed in a shootout with police on Saturday after killing 49 and injuring dozens more after opening fire on a gay nightclub

When he was picked up at the end of the day, another student says he saw Mateen's father slap in across the face in front of the school

Another student says that Mateen went on to taunt others on the bus by making plane sounds

While the nation watched in horror as two hijacked planes crashed into the World Trade Center towers on September 11, 2001, the future Orlando gunman was openly cheering, former classmates say.

Omar Mateen, 29, was shot dead in a gunfight with police Saturday night after opening fire inside an Orlando gay nightclub, killing 49 people and injuring dozens more.

Just hours after the incident, former classmates of the gunman started talking about the massacre in a private Facebook group, going over their memories of Mateen to see if there were any clues of the man who he would become.

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Former classmates of Orlando gunman Omar Mateen (left) say he started cheering in class when the September 11, 2001 attacks happened. Mateen, 29, was a sophomore in high school at the time

One incident in particular stuck out to at least three of the former students, and that was the bizarre way that Mateen allegedly cheered on the terrorists during the September 11, 2001 attacks.

At the time of the attacks, Mateen was a sophomore in high school, attending a Spectrum Alternative School, a school for students with poor grades or behavioral issues.

The Washington Post reports that Mateen's class was watching broadcast video of the first plane hitting one of the towers that morning when the second plane hit on live TV.

After the second plane hit, a former classmate recalls that Mateen 'stood up in class and...started jumping up-and-down cheering on the terrorist'.

One classmate recalls that Mateen was suspended or expelled from the school after his behavior on 9/11. Above, the alternative school in Stuart, Florida that Mateen attended at the time of the attacks

'Mateen was smiling. It was almost like surreal how happy he was about what happened to us,' the student recalls.

Mateen was smiling. It was almost surreal how happy he was about what happened to us.

The former student, who spoke anonymously to the Post for fear of his business clients learning that he attended an alternative school, said Mateen also claimed that Osama bin Laden was his uncle.

Another former classmate told the Post that Mateen started acting out after the two towers were hit, and that he was sent to the dean's office with Mateen 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 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.'

Robert Zirkle, who was a freshman at Martin County High School at the time but who rode the same bus as Mateen, says that Mateen began taunting other students on the ride by making plane noises.

Zirkle says he believes Mateen was suspended or expelled from his school shortly after 9/11.

On the day of 9/11 itself, another student recalls seeing Mateen's father pick him up from school and slap him across the face in public.

'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.'

Mateen's lasting fascination with terrorists was obvious when he called 911 from inside the Pulse nightclub on Sunday.

FBI Director James Comey saidin a statement on Monday that the shooter was involved in precisely three calls to a 911 dispatcher at approximately 2:30am.

Comey said: 'He (Mateen) called and hung up. He called again and spoke with the dispatcher and hung up.

'And the dispatcher called him back and they spoke briefly.

'During the calls he said he was doing this for the leader of ISIS, who he named, and pledged loyalty to. But he also appeared to claim solidarity with the perpetrators of the Boston Marathon bombings and solidarity with the Florida man who died as a suicideb bomber in Syria for Al-Nusra Front - a group in conflict with the so-called Islamic State.