The US justice department has opened an inquiry into how information about President George Bush's secret spying programme was leaked, officials say.
Bush says monitoring is part of his so called war on terror
The investigation is expected to focus on how the New York Times newspaper obtained the information.
Earlier this month, the paper reported that the National Security Agency had been conducting surveillance in the US without warrants.
Mr Bush later admitted he authorised the programme after the 9/11 attacks.
He said people "with known links to al-Qaeda and related terrorist organisations" were monitored.
He has made it clear there is no plan to stop the eavesdropping although the programme is reviewed every 45 days.
The president has suggested the New York Times report which broke the news of the spying was irresponsible.
He said it meant the enemies of the US had "learned information they should not have".
But senators from both the Republican and Democratic parties have expressed concern about the "inappropriate" and "Big Brother" monitoring programme.
President Bush defends it as critical to saving American lives.
He said some of the 11 September hijackers inside the US had communicated with associates outside before the attacks - but the US had not known that until it was too late.
The New York Times reported earlier in December that Mr Bush had signed a secret presidential order following the attacks on 11 September 2001.
It allowed the National Security Agency to track international telephone calls and e-mails of hundreds of people, without referral to the courts.