Many of those arrested in connection with September 11 have spent time in London, where they are said to have become "radicalised".
The Spanish investigative judge, Baltasar Garzon, has named a Muslim cleric, Abu Qatada, as being al-Qaeda's "spiritual leader" in Europe. His bank account has been frozen, and there is a possibility of an extradition request from Spain.
After being criticised abroad for becoming a "haven" for terrorist suspects, the UK Government published a package of emergency legislation that includes the right to indefinitely detain foreign suspects who cannot be deported because of the risk they might be tortured or put to death in the countries that want them.
Some other suspects have connections to a mosque in North London, where the imam is the radical cleric Abu Hamza al-Masri. Born Mustafa Kemal, Abu Hamza was a nightclub bouncer and an engineer before volunteering to fight against the Soviet Army in Afghanistan, where he is said to have lost both hands and an eye. After spending time in Yemen he went to Britain, where he has citizenship. By 1996, he was the figurehead of the Finsbury Park mosque and openly supported Islamic extremism.