The inquiry focuses on disclosures to The New York Times about warrantless surveillance conducted by the National Security Agency since the 11 September 2001 attacks, officials said.
The newspaper recently revealed the existence of the programme in a front-page story that acknowledged that the news had been withheld from publication for a year. The delay, the newspaper said, partly was at the request of the administration and partly because the newspaper wanted more time to confirm aspects of the programme.
On page one
White House spokesman Trent Duffy said that the Justice Department undertook the action on its own and that George Bush, the US president, was informed of it on Friday.
"The leaking of classified information is a serious issue. The fact is that al-Qaida's playbook is not printed on page one, and when America's is, it has serious ramifications," Duffy told reporters in Crawford, Texas, where Bush was spending the holidays.
Catherine Mathis, a spokeswoman for the Times, declined to comment.
"Rather than focus on whether the president broke his oath of office and broke federal law, they are going after the whistle-blowers"
American Civil Liberties Union
Disclosure of the secret spying programme two weeks ago unleashed a barrage of criticism of the administration.
Some critics accused the president of breaking the law by authorising intercepts of conversations, without prior court approval or oversight, of people inside the United States and abroad who had suspected ties to al-Qaida or its affiliates.
Anthony Romero, executive director of the American Civil Liberties Union, said: "It's pretty stunning that, rather than focus on whether the president broke his oath of office and broke federal law, they are going after the whistle-blowers,".
Scott Silliman, a Duke University law professor, agreed that the Justice Department is taking the wrong approach.
"Somebody in the government has enough concern about this programme that they are talking to reporters," Silliman said. "I don't think that is something the Justice Department should try to prosecute."
Bush, who acknowledged the programme's existence and described how it operates, has argued that the initiative is legal in a time of war.
The inquiry launched on Friday is the most recent effort by the Bush administration to determine who is disclosing information to journalists.
Two years ago, a special counsel was named to investigate who inside the White House gave reporters the identity of CIA operative Valerie Plame, an effort that led to perjury and obstruction of justice charges against Vice President Dick Cheney's top aide, Lewis "Scooter" Libby.
More recently, the Justice Department has begun examining whether classified information was illegally disclosed to The Washington Post about a network of secret CIA prisons in Eastern Europe and elsewhere.