The young Saudi said he had arrived in Orlando to meet a friend. But when pressed for details by an alert immigration inspector, "his story fell apart," says one law-enforcement official. The inspector put the Saudi on a flight out of the country. That incident, in late August 2001, was fateful. The FBI has since concluded that the would-be visitor, who carries the common Saudi name of al-Qahtani, may well have been the elusive "20th hijacker" who was supposed to be aboard United Airlines Flight 93, which crashed in the fields of Pennsylvania on the morning of 9/11.

The story of al-Qahtani is one of several new details of the 9/11 plot uncovered by the federal panel probing the terrorist attacks. Law-enforcement sources tell NEWSWEEK the story has some surprises. At the time the inspector turned al-Qahtani away, there was no sign he was connected to terrorism. But after 9/11, agents began looking into other Mideasterners who had tried to enter the United States in the preceding months and soon focused on the Orlando, Fla., incident. One item grabbed their attention: 9/11 ringleader Muhammad Atta was at the Orlando airport that same day and made a call on a pay phone to a Mideastern country--apparently concerned that his guest hadn't gotten off the airplane. A surveillance camera captured Atta making the call. Months later, a Saudi by the same name--al-Qahtani--was captured by U.S. troops in Afghanistan and flown to Guantanamo Bay, Cuba, where investigators later ID'd him as the man in Orlando.

Some federal officials are eager to talk about the Orlando incident, saying it illustrates that at least one federal inspector was on the ball prior to 9/11. (The inspector, a 58-year-old Vietnam vet named Jose Melendez-Perez, is slated to be the star witness at a hearing of the 9/11 panel next week.) Investigators believe that while al-Qahtani (whose first name has not been disclosed) was not the only Qaeda operative who tried and failed to join the 9/11 plot, he may have been the last. His rejection in Orlando explains why Flight 93 had only four hijackers aboard (the three other hijacked planes each had five). The smaller size may have enabled the plane's passengers to battle the hijackers, forcing Flight 93 to crash rather than hit its intended target--either the White House or the Capitol. But investigators say the incident also raises new questions about why there wasn't more follow-up into what al-Qahtani was up to and why his case wasn't pieced together with other developments that summer, including the arrest in Minneapolis the week before of Zacarias Moussaoui. The Feds once thought Moussaoui was the 20th hijacker--a theory they quietly abandoned last year when they concluded al-Qahtani was more likely the real one.