Some immigrants used false documents to enter the United States, others let their legal visas expire once in the country. And at least 21 foreign nationals became naturalised US citizens before being charged or convicted as terrorists.

Abu Musab al-Zarqawi, the terrorist leader allegedly claiming responsibility for the graphic beheading of a US civilian in Iraq, is adopting an increasingly public and influential role among Islamic militants, US officials and terrorism experts say.

Saddam Hussein has joined terror chief Osama bin Laden at the top of the list of America’s most wanted men, US officials said today.