U.S. officials wouldn’t say where the physical evidence was discovered. But they did acknowledge that one key source for intelligence was Ali Al Ghamdi, Al Qaeda’s reputed leader in Saudi Arabia and a suspected mastermind of the May 12 Riyadh bombings. Al Ghamdi, also known as Abu Bakr, gave himself up to Saudi authorities in late June. Officials indicated Al Ghamdi’s information about the threat was corroborated by other sources, including electronic intercepts. Some officials caution, however, that, like other Qaeda captives, Al Ghamdi could be making up the threats to fool or curry favor with interrogators. Officials also say public disclosure of the hijacking threats, which supposedly were to be carried out in late summer, may have disrupted the terrorists’ plans.

The warning suggested hijackings could occur at several locations around the globe, including the Eastern United States, Britain, Italy or Australia. Several U.S. officials told NEWSWEEK their understanding was that one reason Al Qaeda was still interested in hijackings was that the bin Laden network always liked to “return to the scenes of the crimes”–and particularly to finish off targets that it might have missed in earlier attacks. One target the terrorists might want to revisit is the Capitol building. A law-enforcement official said that investigators were “99 percent certain” that the hijackers who commandeered the 9/11 plane that crashed near Pittsburgh intended to crash it into the Capitol. A spokeswoman for the Capitol Police said her agency was aware of possible Qaeda designs on the Capitol and that security in the complex was on heightened alert even before the latest threats surfaced.