Early Sunday morning, Omar Mateen, 29, carried out a gruesome terrorist attack at a popular gay nightclub in Orlando, Florida. Mateen made a call to 911 and pledged his allegiance to the Islamic State during the massacre, which left 49 dead and 53 injured. ISIS later claimed credit for the attack, issuing a statement through a “news agency” affiliated with the terror group.

Since the barbaric attack, much has been learned about the radical Muslim who despicably targeted and murdered innocents on American soil in the name of his ideology: radical Islam.

Here are seven things you need to know about Mateen:

1. Mateen was a United States citizen radicalized in America.

Mateen was born in New York to Afghan parents. He later became a resident of the state of Florida, where he attended high school and would later carry out his vicious terrorist attack.

Mateen’s family clearly has ties to radical Islam, certainly his father, but FBI Director James Comey says he is “highly confident” that Mateen was radicalized, at least in part, via the internet.

2. Mateen was expelled from his Florida high school for celebrating the 9/11 terror attacks.

According to reports from former high school classmates, Mateen, who was a sophomore at the time, celebrated the horrific acts of terrorism committed against America on September 11, 2001. The mass-murderer allegedly cheered on the terrorists as he and his classmates watched the second plane slam into one of the Twin Towers. He also made “plane noises” and acted as though he was “running into a building” on the bus ride home.

Former classmate Robert Zirkle says that Mateen was expelled from Martin County High School after he celebrated the 9/11 attack and then continued to make “other comments in classes” as well.

Another former classmate allegedly told Zirkle 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.”

“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,” witnessed yet another former classmate, reports The Washington Post.

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

Moreover, Zirkle and a different unnamed former classmate claim that Mateen bragged that Osama bin Laden was his uncle.

3. Mateen was investigated by the FBI twice prior to the Orlando Massacre

According to The Daily Beast's source, “Mateen became a person of interest in 2013 and again in 2014. The Federal Bureau of Investigation at one point opened an investigation into Mateen, but subsequently closed the case when it produced nothing that appeared to warrant further investigation.”

Mateen was once investigated for “suggesting an affinity for Islamist groups, and a second time for vague connections to another Florida man who traveled to Syria to become a suicide bomber,” reports The Washington Post.

The law-enforcement source via The Daily Beast said Mateen was “a known quantity.” He added, “He’s been on the radar before.”

Despite the suspicion and the FBI investigation, “Mateen had been employed since 2007 as a security guard with G4S, a large multinational security company that provides services to more than two-dozen juvenile-detention facilities in Florida.”

4. As Mateen pledged his allegiance to the Islamic State, he referenced the Boston Marathon bombers.

As Mateen was barbarically slaughtering club-goers on Sunday morning, he made a call to 911 and pledged his allegiance to the Islamic State. During the phone call, Mateen referred to Boston Marathon bombers Dzhokhar and Tamerlan Tsarnaev, reports Boston.com.

He allegedly bragged about his connection to the Boston terrorists prior to the attack on the gay nightclub, as well. The admission led to Mateen being investigated by the FBI, but no links were found.

ISIS has instructed their followers to carry out attacks during Ramadan; Mateen was seemingly following their orders. Despite the connection, Mateen is still considered a “lone wolf” attacker at the moment since he has no known direct connections to the Islamic State.

5. Mateen may have been living a double life as a gay man.

Mateen apparently used gay dating apps and frequented gay nightclubs, according to witnesses.

“Kevin West, a regular at Pulse nightclub, said Omar Mateen messaged him on and off for a year before the shooting using the gay chat and dating app Jack’d,” reports the Los Angeles Times.

At least four regular customers of Pulse, the lesbian, gay, bisexual and transgender nightclub where the massacre took place, told the Orlando Sentinel on Monday that they believed they had seen Mateen there before.

"Sometimes he would go over in the corner and sit and drink by himself, and other times he would get so drunk he was loud and belligerent," said Ty Smith, a frequenter of the gay nightclub.

Smith said he saw Mateen at least a dozen times at Pulse.

According to the New York Post, Mateen’s ex-wife says she believes that Mateen was gay.

6. The murderer’s father Seddique Mateen—a radical Muslim who has voiced homophobic and anti-American views—claims that his son was recently enraged at the sight of two gay men kissing

As The Daily Wire reported, Seddique is a radical Muslim who declared that homosexuals will be “punished” the day following his son’s murder-spree at a gay nightclub:

Monday morning, just one day after his son's terror attack on the Orlando LGBT community, Seddique Mateen posted a video on Facebook in which he declared that "God will punish those involved in homosexuality," but adding that it is "not an issue that humans should deal with."

Further, the murderer’s father also vocalized anti-American sentiments frequently:

CBS News reports that Seddique, an Afghan granted asylum in the United States, "has well-known anti-American views" and "regularly attended Friday prayers at a Florida mosque with his son."

Seddique, rejecting any ties to his son’s actions and Islam, claims that his son was recently infuriated at the sight of gay men kissing.

"We were in Downtown Miami, Bayside, people were playing music. And he saw two men kissing each other in front of his wife and kid and he got very angry," said Seddique on Sunday. "They were kissing each other and touching each other and he said, 'Look at that. In front of my son they are doing that.' And then we were in the men's bathroom and men were kissing each other."

7. Mateen and his wife reportedly scoped out Walt Disney World as a possible venue for a terror attack.

According to The Daily Mail, Mateen “considered making one of the biggest tourist sites in the world the site of his deadly massacre - Walt Disney World.”

Omar Mateen and his wife Noor Zahi Salman visited the theme park in April claims a federal law enforcement source who said that Mateen was looking at Downtown Disney and the gay club Pulse.

There have been no charges filed against Salman and it is not clear if they took their 3-year-old son with them when they went to scope out the park.