Watching “The Looming Tower”? Meet the Real John O’Neill.

When the Twin Towers fell on Sept. 11, 2001, among the thousands killed was the one man who may have known more about Osama bin Laden and Al Qaeda than any other person in America: John O’Neill.

The FBI’s leading expert on Al Qaeda, O’Neill had sounded the alarm about the group’s threat to the United States long before 9/11.

His story is dramatized as part of the new Hulu series, “The Looming Tower,” in which he’s played by Jeff Daniels. But you can meet the real John O’Neill in the classic FRONTLINE documentary The Man Who Knew — which is streaming online, for free, above.

Drawing on exclusive interviews with many of O’Neill’s closest friends and associates, The Man Who Knew tells the story of a controversial figure within the U.S. intelligence community — a man who was obsessed with Osama bin Laden long before most people had ever heard of him, but whose polarizing style made him too much of a maverick for some at FBI headquarters.

As the documentary explores, O’Neill had investigated the bombing of the American embassies in Africa, the USS Cole in Yemen, the Khobar Towers in Saudi Arabia and the first attack on the World Trade Center in 1993. He became increasingly convinced that another attack on U.S. soil was imminent — and the film traces his growing frustration with Washington’s attitude toward the possibility that Al Qaeda sleeper cells were operating in the U.S.

“What John O’Neill was trying to do was get a momentum going in the FBI to look seriously for those cells,” says Richard Clarke, the nation’s former counterterrorism czar, in the documentary.

O’Neill’s brash approach created friction with FBI leadership. Following bureau investigations into his temporary loss of a briefcase containing a classified report and charges that he used an FBI car to give a ride to his girlfriend, O’Neill left the job he loved and entered the private sector in the summer of 2001 — as director of security for the World Trade Center.

His body was found in the debris of the South Tower.

“The night before he died, he had said to me, ‘We’re due, and we’re due for something big,'” his friend Jerry Hauer, New York City’s former director of emergency management, tells FRONTLINE in the documentary.

Watch The Man Who Knew, from FRONTLINE’s Michael Kirk, above.

Correction: An earlier version of this story misstated when John O’Neill became director of security at the World Trade Center. O’Neill began his work there in 2001.