Washington ? Just one month before the Sept. 11, 2001 terrorist attacks, the so-called 20th hijacker may have been prevented from entering the country by an alert border agent at Orlando International Airport, federal investigators said Monday.

Sometime in August 2001, a man known only by his last name — al-Qahtani — arrived at the airport on the same day that lead hijacker Mohamed Atta was known to have been there and to have used a pay phone.

But al-Qahtani was turned away after questioning by border agent Jose Melendez-Perez, who is currently an inspector with the Customs and Border Protection arm of the U.S. Department of Homeland Security. Melendez-Perez is scheduled to testify about the “incident in Florida” at a Monday hearing of the National Commission on Terrorist Attacks Upon the United States, better known as the 9-11 Commission, according to a witness list.

Al Felzenberg, the commission’s spokesman, would not confirm details of the incident, but said Melendez-Perez’ quick thinking might have helped prevent the Sept. 11 terrorist attacks from being even more devastating. It’s long been suspected that a so-called “20th hijacker” was supposed to have been part of the terrorist team that was overwhelmed by passengers aboard United Flight 93 that crashed in Pennsylvania.

It’s been speculated that that flight may have been aimed at the U.S. Capitol or the White House, and the fact that the terrorist team was one man short may have given passengers an edge.

“He helped make it a lot less of a tragedy than it might have been,” said Felzenberg. “There were many people who worked for the government who helped enhance security, and he’s one of them.”

At least one other of the Flight 93 hijackers — Saeed Alghamdi — is known to have entered the country through Orlando on June 27, 2001, according to previous FBI testimony. A total of four hijackers entered through Orlando en route to joining terrorist cells in South Florida, the FBI has said.

A congressional source said the would-be hijacker al-Qahtani was later picked up in Afghanistan as a combatant and is now being questioned at the U.S. Naval Base at Guantanamo.

The Orlando incident was first reported in the edition of Newsweek that went on sale Monday.