The US Justice Department has opened a probe into the leaking of classified information which revealed that US President George W. Bush had authorized a secret government wiretap program, an official source said on Friday.
"We have opened an investigation into the unauthorized disclosure of classified information related to the NSA [National Security Agency]," said a Justice Department official who requested anonymity.
The probe was opened after Bush earlier this month urged a "full investigation" into who leaked information that he had authorized the NSA to monitor massive volumes of telephone and Internet communications.
However, the White House said on Friday it had no role in the decision to investigate the leak.
"The Justice Department undertook this action on its own, which is the way it should be," White House spokesman Trent Duffy told reporters in Crawford, Texas, where Bush was enjoying a year-end vacation on his ranch.
Rights groups, however, quickly branded the investigation a crackdown on critics of the president.
"President Bush broke the law and lied to the American people when he unilaterally authorized secret wiretaps of US citizens," said Anthony Romero, executive director of the American Civil Liberties Union.
"Our nation is strengthened, not weakened, by those whistleblowers who are courageous enough to speak out on violations of the law," Romero said.
Domestic spying is a sensitive issue for many in the US who are proud of their civil liberties.
Similar revelations about domestic spying led to legislation in the 1970s that allows wiretapping but requires government agencies to obtain a special court warrant for it.
While the source of the leak remains unknown, media reports have suggested that some agents were concerned about the program's legality.