State security guard licensing photos of Omar Mateen since 2007. (Florida Department of Agriculture and Consumer Services)

FORT PIERCE, Fla. — Omar Mateen watched Islamic State terrorist videos — including some showing brutal beheadings — and talked to co-workers about them, according to government and congressional sources briefed by the FBI on the investigation into the massacre at the Pulse nightclub in Orlando.

The discovery that Mateen, who was born in New York to Afghan parents, watched Islamic State videos was one piece of the evidence behind FBI Director James Comey’s statement on Monday that the perpetrator of the nightclub massacre had undergone “radicalization.”

Sources familiar with a classified briefing that Comey and National Counterterrorism Center Director Nicholas Rasmussen gave to House members this week said Mateen had also told his co-workers that he wanted to become a “martyr” in an operation. If the FBI “raided his house and killed his wife and child, that would free him to martyr himself in an operation,” Mateen had told co-workers, according to two sources who attended the briefing.

That and other comments Mateen made — including that he had obtained a car from another “martyr” (an apparent reference to American suicide bomber Moner Mohammad Abusalha ) so alarmed co-workers in 2013 that they reported it to the local sheriff’s office, which in turned alerted the FBI. The bureau then launched a “preliminary investigation” into Mateen that lasted 10 months — six months longer than the standard preliminary probe — as FBI agents struggled to figure out what to make of Mateen.

But other evidence emerging in the investigation also points to a troubled man who was experiencing psychological problems, even as he expressed solidarity with Islamic terror groups.

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Mateen, a licensed security guard for nearly seven years, also became addicted to steroids that he took to enhance his ability in weightlifting, which was one of his hobbies. He talked to colleagues about how he was able to “beat” his addiction to the drugs, said one congressional source who attended the Tuesday briefing by Comey and Rasmussen.

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The FBI is also talking to witnesses who say Mateen had previously attended the Pulse nightclub, but the bureau has so far reached no conclusions about the visits, including whether he may have been seeking to meet gay men or might have been casing the club for his future attacks.

Federal investigators published a poster Wednesday seeking information about Orlando nightclub gunman Omar Mateen. (FBI)

“He was clearly confused,” said one congressional source who attended a briefing with Comey for House members. But it is still far from clear to what extent Mateen was influenced by radical postings and videos on the Internet.

One trail the bureau is pursuing is Mateen’s two trips to Saudi Arabia in March 2011 and March 2012. But how important they were in Mateen’s radicalization is also unclear; both trips appear to have been for pilgrimages and may only be evidence of his Islamic faith, said sources familiar with the investigation.

Comey has defended the FBI’s decision to terminate its investigation in 2013 after Mateen told agents in two interviews that he only made the statements that alarmed co-workers “in anger” because he was being teased as a Muslim. Sources who attended the Comey briefing say the FBI also discovered that some of what Mateen had told his colleagues was untrue. For example, agents found that, contrary to what he had said, Mateen did not get his car from suicide bomber Abusalha or any other Islamic martyr but from a relative.

Still, some House members who attended the briefing say they are now convinced that the FBI had enough to continue its investigation and keep Mateen on the terror watch list. “Between the trips to Saudi Arabia and the lies he told, they should have kept him on,” said one House member, who asked not to be identified talking about a classified briefing.

Mateen, 29, died early Sunday during a shootout with police after opening fire on the crowded gay nightclub near downtown Orlando. One hundred people were shot, 49 of them fatally.

“I would call it a hate crime. I would call it terrorism. It’s both,” Orlando FBI Special Agent Ron Hopper said Wednesday when pressed about what motivated the attack.

The FBI published a poster on Wednesday seeking more information about Mateen, who lived in Fort Pierce with his wife and young son.

“We need your help in developing the most complete picture of what the shooter did and why he did it,” Hopper said.

The shooting, which was the deadliest mass shooting in U.S. history, came several years after federal investigators questioned Mateen in 2013 and 2014 for possible ties to radical Islamists. The FBI dropped Mateen from the terror watch list after no “derogatory” information was found, the agency said.

Information regarding his time on the terror watch list never reached Florida authorities, who renewed Mateen’s license to work as an armed security guard last August.

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“He successfully completed the application, had a criminal background check, and there is nothing in that record that would have disqualified this individual, who is a U.S. citizen who had a clean criminal record … from receiving those licenses,” Florida Agriculture Commissioner Adam Putnam told reporters on Monday.

The Florida Department of Agriculture and Consumer Protection, the agency that licenses security guards, turned over more than 60 pages of records and 10 photos from Mateen’s original 2007 application and subsequent renewals. The office released the documents in response to public records requests filed by Yahoo News and other media outlets.

Mateen held both a “D” security guard license and a “G” security guard firearms license, which enabled him to carry two guns specified by the state at any one time.

Investigators have said Mateen was firing an assault-style rifle and a Glock handgun — neither of which he was licensed to carry at work — when he ambushed the crowded Pulse nightclub early Sunday. The handgun Mateen used at work was found in his car outside the club.

Mateen received an associate degree in criminal justice technology in 2006 from Indian River State College in Fort Pierce, Fla. He hoped to work as a police officer but reportedly failed the state licensing exam. His desire to be a police officer seems to be reflected in the email handle — “onpatrol1986” — he listed on the security guard applications.

Mateen graduated from an alternative high school in Stuart, Fla., in 2003. He worked in retail and at Gold’s Gym and nutrition stores before landing a job as a prison corrections officer in October 2006. That gig lasted only seven months before Mateen was “administratively dismissed,” according to the Palm Beach Post.

Florida’s application for armed security guards asks about mental illness and criminal history but not whether the applicant was ever investigated for terrorist ties. Mateen’s ex-wife, Sitora Yusufiy, has told reporters that the shooter may have been mentally ill.

According to the state applications, Mateen passed a physical with a Fort Pierce general practitioner and a written mental exam and evaluation with a South Miami psychologist in September 2007.

Mateen has been employed with G4S Secure Solutions (formerly Wackenhut) since 2007. His assignment at the time of the shooting was working the security checkpoint at an upscale golf subdivision in Port St. Lucie, Fla.

(Jason Sickles reported from Fort Pierce, Fla.; Michael Isikoff from Washington, D.C.)