The FBI has served warrants on a number of internet service providers (ISPs) in a bid to turn up more evidence on the US terrorist attacks.

According to reports, the Bureau has approached a number of US ISPs, including Earthlink, AOL and Yahoo, allegedly following a lead involving an email connected to the attacks. It is believed that the investigators have demanded access to logs to see if the email address has passed through the ISPs' networks at any time.

There are also rumours that the FBI has been requesting ISPs to install controversial Carnivore wiretapping equipment to dig for more information. But this has not been confirmed by any of the ISPs involved.

The FBI is able to demand limited access to an ISP's data under the Foreign Intelligence Surveillance Act (FISA). Typically the Act prevents abuse of power and activities that could be classed as spying.

But a warrant can be obtained in cases where information may have been leaked or obtained by a foreign government. Only directors in the CIA and FBI and the secretaries of state and defence can request a FISA warrant.