An Editor’s Note published July 12, 2017, about Kevin Deutsch’s reporting appears at the end of this story. 

The headline of this story has been corrected. For more information, see the note at the bottom of the story.

PORT ST. LUCIE, FLA. — Omar Mateen’s anti-American ideology was evident to residents here as early as Sept. 11, 2001, when the New York City-born terrorist cheered the collapse of the World Trade Center while watching the attack on TV in his high school classroom, a childhood friend said.

Aahil Khan, 28, of Fort Pierce, Florida, said he used to hang out with Mateen after school when the now infamous gunman attended Martin County High School. He recalls Mateen telling him he’d been suspended for “cheering” in class as Mateen and his classmates watched the 9/11 attacks unfold on TV.

“He said some of the kids beat him up” after school for lauding the acts of terror.

“He didn’t hide it,” Aahil Khan said of Mateen’s happiness at attacks on New York and Washington, D.C. “He said they suspended him because he was yelling to everyone that it was a good thing. He was the only one there who wasn’t upset that they attacked.”

“I feel like that was a big moment in his life, seeing” the attacks.

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Khan said it was clear even then that Mateen was mentally ill.

“I felt sorry for him,” said Khan, who lost touch with Mateen after high school and believed his former friend was depressed, somewhat delusional and paranoid. “He couldn’t control his emotions.”

Khan’s account was among a number of recollections shared Monday by former friends and fellow worshippers who knew the Orlando gunman, a man one federal law enforcement source with knowledge of the investigation called an apparent “classic lone wolf” terrorist inspired by the Boston Marathon bombers, the Islamic State group and terrorists like radical Islamist cleric Samir Khan, formerly of Long Island, who was killed in an American drone strike in Yemen in 2011.

A search of Mateen’s Fort Pierce apartment by the FBI uncovered other evidence of his radicalization — including online records and notes about jihadist ideology, the source said. The residence also contained run-of-the-mill items like clothes and housewares, as well as Batman, Spider-Man and Ninja Turtles toys — trappings of Mateen’s life as a middle-class, married father of a 3-year-old boy. A number of books about the Muslim faith and Islamic texts were also found in the apartment, he said.

Mateen appeared devout, those who knew him said. When he wasn’t working as a private security guard for G4S, the global security firm formerly known as Wackenhut, Mateen often spent time attending prayer services at the Islamic Center of Fort Pierce or lifting weights at Gold’s Gym in Port St. Lucie.

Members there said he worked out several times a week, rarely interacting with other weightlifters.

“He was definitely a strong kid,” said gym-goer Jason Hart of Port St. Lucie. “He always looked intense. Like he was training. It’s kind of eerie to think about that now.”

Mateen would sometimes come to prayer services straight from the gym, changing from workout clothes into traditional Islamic garb. Other times, he had his son in tow. Mateen’s sisters, father and other relatives also regularly visited the center, worshippers there said. He last attended prayer services on Friday, they said.

“He would bring his son and pray, usually, always in and out,” said Saeed Afzal, of Fort Pierce, who attends services there. On Friday, Mateen attended alone and stayed longer than usual, Afzal said.

“No one knew he would do this.”

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Investigators learned over the weekend that Mateen legally purchased weapons used in the attack at the St. Lucie Shooting Center and also attempted to buy body armor at a store in the area before the shooting, the source said.

On Monday, the shooting center’s owner, a former NYPD detective named Ed Henson, told reporters Mateen purchased a long gun and a handgun at his store about 10 days before.

“An evil person came in here and they legally purchased two firearms from us, and if he hadn’t purchased them from us, I’m sure he would have gotten them from another local gun store,” Henson told reporters outside his business.

“He passed the background check that every single person that purchases a firearm in the state of Florida undergoes.”

Mateen appeared comfortable using firearms, the source said, having fired at shooting ranges in the area, and obtained a gun permit for his security work.

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At one time, investigators believe, Mateen wanted to be a police officer.

He received his associate of science degree in criminal justice technology from Indian River State College in 2006, the school confirmed Monday. He also worked briefly as a state correction officer in 2006 and 2007, officials said.

Before moving to Florida, Mateen — who records show was born in New Hyde Park — and his family lived in Westbury and Flushing, Queens, records show.

Mateen’s father, Seddique Mateen, spoke to reporters at his Port St. Lucie home Monday, saying of Saturday’s mass shooting: “I was in shock. My whole family was in shock.

“As a father, I don’t want any father to go through what we are going through.

“What he did was completely an act of a terrorist.

“I condemn what he did. I wish I did know what he was doing . . . ,” he added. “I’d have arrested him myself.”

CORRECTION: This story previously appeared with an incorrect headline that identified a source differently from what appeared in the story. The story identifies the source as a friend, not a classmate. This story was written by former Newsday reporter Kevin Deutsch; the headline was not. 

Editor’s note: Newsday undertook an extensive, four-month review of reporting by Kevin Deutsch, who covered law enforcement from April 2012 to September 2016. 

The review of the former Newsday reporter’s work began after The Baltimore Sun this year reported that law enforcement and other officials questioned the veracity of Deutsch’s nonfiction book “Pill City” about Baltimore’s drug trade. In addition, questions arose about individuals named in Newsday stories by Deutsch. Book publisher St. Martin’s Press and Deutsch have said they stand behind the book. 

We are dedicated to accurate, factual reporting, to the highest journalistic standards and to maintaining our credibility with Newsday readers. We also are committed to being accountable to our readers. Newsday undertook the detailed review in that spirit and because of the concerns that were raised.  

In late February, as our review was under way, The New York Times reported in an editor’s note that The Times “had been unable to locate or confirm the existence of two people who were named and quoted” in a Dec. 29, 2016, freelance article written by Deutsch. Deutsch “maintains that the interviews and the descriptions are accurate,” The Times wrote. 

Newsday reviewed 600 stories with reporting by Deutsch. We contacted officials in the police departments regularly involved in Deutsch’s coverage. They said they had not had problems with his work. We then focused our research and reporting on individuals who, as described in the stories, would not be considered officials, or well-known, public figures. 

The review found 77 stories with 109 individuals from Deutsch’s reporting whom Newsday could not locate. The main points of the stories were not affected. While two stories about the Orlando nightclub shooter Omar Mateen were based on sources Newsday could not locate, other media reported the main points of those stories but with attribution from different sources.  In this story, Newsday could not locate: Aahil Khan, Jason Hart and Saeed Afzal. This story’s description of Mateen’s response to Sept. 11, 2001, and several of his affiliations were also reported by other media with attribution to different sources.   Newsday is attaching an editor’s note to each story online that contains individuals we cannot locate. 

Here’s how Newsday conducted the review:  

Researchers and reporters searched local and national public records, sites providing nationwide people searches, databases of business, real estate and conviction records, social media sites including Facebook, LinkedIn and Ancestry.com and nationwide news archives. They searched potential alternate spellings and other name variations. Their reporting followed potential leads they found through research, within stories and in information shared by Deutsch during the review. 

Finding people after publication, in some cases years later, can be difficult because of changes in residence, circumstance and contact information. Some may not have given their real names. 

On the law enforcement beat, reporters may encounter people who lead lives that are not reflected in public records or other sources of information that would help locate them. It is possible that some on our list were difficult to find or reluctant to respond to our review because they are undocumented immigrants, those battling or recovering from addiction or people involved in or around illegal activity.  

Some on our list were described discussing crimes in their neighborhoods, and others as relatives, friends or neighbors of victims or as individuals living near or knowing those accused of crimes. 

Others we have not been able to locate, though, are described as bystanders, neighbors, spectators, relatives of drug victims, witnesses to news events or related in some way to people in the news. Still others are described in stories as people actively engaged in public issues, such as activists, protesters and marchers. Many individuals on the list are described as local. 

Deutsch said in email exchanges with Newsday that “I have no doubt about the veracity of the claims of the sources I quoted.” He also said, “Not a single public official, source, or other interviewee has raised any issues with even one of these stories.” 

“It's impossible for any reporter to know whether the name given to him by interviewees on the street--or those reached briefly by phone or email-- is that person's full and legal name, rather than an alias or variation of their real name (maiden names and certain common nicknames/abbreviations for first names are often published by newspapers, including Newsday.). But every one of the names on Newsday’s list was the name given to me by that interview subject, verbatim.” 

During the four months of our review, Newsday shared questions and updates with Deutsch as we progressed in the search for individuals we could not locate. We requested notes and contact information. Deutsch sent us notes he said represented all individuals we were unable to locate and responded over the course of the review by email, sharing information he said was from his recollection and notes. 

Reporters followed up on all information shared by Deutsch. He did not provide contact information for those on our list. Newsday reporters and editors sought unsuccessfully several times to meet with Deutsch to discuss his reporting and to review his notes together to ensure we were not missing contact information or other details that might help locate individuals. Deutsch maintained that the notes he shared “serve as evidence of interviews” with each source.  

Deutsch said he kept contact information in a Rolodex he left behind at Newsday’s main office and in a company-issued cellphone he returned within a week after resigning on Sept. 6, 2016. Editorial staff did not find a Rolodex or other notes at our office, but found notes left at Newsday’s desk at a courthouse pressroom where he worked. We shared them with Deutsch and he confirmed they were his. As per company policy, the contents of the cellphone had been deleted immediately after Deutsch returned it to Newsday. 

Maintaining the trust of our readers is essential to our mission.  If we are able subsequently to locate any individuals, we will update our stories.