EXCLUSIVE: Why FBI Agents Let Pulse Nightclub Killer Omar Mateen Go Free Before His Slaughter at Orlando Gay Bar 10 Years Ago

It's been 10 years since a madman walked into a gay nightclub in Orlando and opened fire, killing 49 people and leaving another 53 injured.

But RadarOnline.com has learned the massacre at the Pulse nightclub might have been predicted, as the FBI was reportedly already well-aware of Omar Mateen – and his capabilities for murder.

On June 12, 2016, Mateen orchestrated the then-deadliest mass shooting in modern U.S. history.

Moments after the bullets began to fly, the 29-year-old called 911 and swore his allegiance to ISIS. He further said the U.S. killing of Abu Waheeb in Iraq the previous month "triggered" the shooting.

Mateen was shot and killed by police after a three-hour standoff. The incident was later deemed a terrorist attack by FBI investigators, who some blame for missing clues to the coming carnage.

According to documents reviewed by Radar, the FBI had investigated Mateen in 2013 when he was employed by the security company G4S and served as a security guard at the St. Lucie County courthouse.

A 2013 email from the St. Lucie Sheriff's Department to the Sheriff's Department Director of Detention, with comments from Randall Glass, the Florida regional resident in charge for the FBI, was among the files.

"Rand told me 'We do NOT believe he is a terrorist,'" the email read. "Yesterday, the FBI spoke with him in person, and reportedly, Mateen became very upset that someone contacted the FBI. Regarding this demeanor, Rand said, 'I don't believe he will go postal or anything like that.'"

In the aftermath, the conservative watchdog group Judicial Watch lashed out at investigators, with organization president Tom Fitton blasting: "The documents show the FBI blew it.

"The agency let Mateen off the hook even though he threatened his co-worker and tried to lie to the FBI agents questioning him about terrorist ties."

Bodycam footage from the horrific night showed the massive police response to the shooting, including video and audio of the gunfire, the club in disarray, and an officer shouting to Mateen, "Come out with your hands up or you will die!"

In another video, an officer can be heard announcing that the shooting appears to be an act of terror: "It's terrorism in Orlando. The shooter pledged something to the Islamic State."

Other footage shows officers leading victims out of the building, with one telling a patron escaping a bathroom, "Follow the sound of my voice. Come this way."

One victim, Norman Casiano suffered four bullet wounds during the shooting but survived.

"I couldn't believe it was happening," Casiano said of the massacre, revealing that he and two friends, who unfortunately didn't survive the shooting, were gunned down in the bathroom stalls of the nightclub.

"He laughed, and as he's laughing, that's when he fires through the whole front of the stall," he recalled of gunman Omar Mateen.

When police arrived, Casiano was able to crawl over the bodies and reach safety.

Victim Felipe "Tony" Marrero was also interviewed, remembering Mateen deliberately "blowing up people's heads" during the horrific rampage.

"The only way that I had a chance to survive was to make it seem like I didn't have a head," he said. "But he still shot me in the back."