Officials at the US justice department have opened an investigation into how details of a secret surveillance programme were leaked to US newspapers.
Earlier this month, the New York Times reported details of how the National Security Agency had been conducting surveillance in the US without warrants.
The US President, George W Bush, later admitted he authorised the programme, which monitored international phone calls and emails of people in the US after the 11 September 2001 attacks.
Mr Bush said people with known links to al-Qaeda and related terrorist organisations were monitored.
He said the newspaper report had jeopardised national security.
