Saudi Arabian officials and others have questioned whether some of the hijackers identified by the FBI in the weeks after the attacks used stolen identifications. Mueller said those questions have been answered.
"We at this point definitely know the 19 hijackers who were responsible," he said. "We have been successful in working with our foreign counterparts in identifying places where the conspiracy we believe was hatched as well as others who may have been involved in the conspiracy."
Mueller provided no new information on the hijackers' identities beyond his statement at a briefing Friday for reporters. Neither did he name any of the places abroad where authorities now believe the conspiracy was initiated, or any of the other conspirators.
Attorney General John Ashcroft said last month that three of the leaders of the hijackers and three accomplices were part of a terrorist network that operated at least since 1999 in both Hamburg, Germany, and in the United States.
German authorities previously issued international arrest warrants for the three alleged accomplices, Said Bahaji, a German national, Ramsi Binalshibh, a Yemini national, and Zakariya Essabar of Morocco. All three left Hamburg shortly before the Sept. 11 attacks.
Ashcroft said the three had extensive connections to Mohamed Atta and Marwan Al-Shehhi, the suspected pilots of the hijacked planes that crashed into the World Trade Center in New York, and Ziad Jarrah, suspected of flying the plane that crashed in Pennsylvania.
Authorities in Spain and the Czech Republic have been investigating trips Atta made to both countries earlier this year. Czech officials said Atta met with an Iraqi intelligence agent in Prague in April.
Mueller said the FBI has developed 420,000 leads since Sept. 11, in the investigations of both the hijackings and the mailing of anthrax-laced letters. He issued a new plea to the public for help in solving both cases.
By Karen Gullo
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