] In Washington, the national security agency used Echelon,
] an intelligence system coordinated by the United States
] but involving several of its allies, including the UK, to
] monitor more than 10 mobile phones used by Mohammed.

There are also some other interesting links at the bottom of the page, which trace connections between the hijackers and other members of Al Qaeda.

Echelon Helped Capture 9/11 Planner