The inquiry focuses on disclosures to The New York Times about warrantless surveillance conducted by the National Security Agency since the Sept. 11 terrorist attacks, officials said.

The newspaper recently revealed the existence of the program in a front-page story that also acknowledged that the news had been withheld from publication for a year, partly at the request of the administration and partly because the newspaper wanted more time to confirm various aspects of the program.

White House spokesman Trent Duffy said Justice undertook the action on its own, and Bush was informed of it 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.

Disclosure of the secret spying program two weeks ago unleashed a firestorm of criticism of the administration. Some critics accused the president of breaking the law by authorizing 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.

Bush, who publicly acknowledged the program's existence and described how it operates, has argued that the initiative is legal in a time of war.

The inquiry launched Friday is only 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 I. "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.

The NSA leak probe was launched after the Justice Department received a request from the spy agency.

It is unclear whether Attorney General Alberto Gonzales will recuse himself from the inquiry. He was White House counsel when Bush signed the executive order authorizing the NSA, which is normally confined to overseas operations, to spy on conversations taking place on American soil.

For the past two weeks, Gonzales also has been one of the administration's point men in arguing that the president has the constitutional authority to conduct the spying.

"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 whistleblowers," said Anthony D. Romero, executive director of the American Civil Liberties Union.

Romero said a special prosecutor from outside the Justice Department needs to be appointed. "This confirms many of the fears about Gonzales' appointment - that he would not be sufficiently independent from the president and that he would play the role of a crony," he said.

Duke University law professor Scott Silliman agreed that the Justice Department is taking the wrong approach.

"Somebody in the government has enough concern about this program that they are talking to reporters," Silliman said. "I don't think that is something the Justice Department should try to prosecute."

Douglas Kmiec, a Pepperdine University law professor, said the Justice probe is the next logical step because the NSA is alleging a violation of a law that prohibits disclosure of classified information.

"The Department of Justice has the general obligation to investigate suspected violations of the law," Kmiec said. "It would be extraordinary for the department not to take up this matter."

The NSA probe likely will result in a repeat of last summer's events in Washington, where reporters were subpoenaed to testify about who in the administration told them about Plame's work at the CIA. New York Times reporter Judith Miller spent 85 days in jail for refusing to reveal her sources.

Lucy Dalglish, executive director of the Reporters Committee for Freedom of the Press, said the Plame investigation was about "political gamesmanship." But, she said, the NSA leak probe is frightening.

"In this case, there is no question that the public needed to know what the New York Times reported," she said. "It's much more of a classic whistleblower situation. The public needs to know when the government is engaged in things that may well be unconstitutional."

The surveillance program bypassed a nearly 30-year-old secret court established to oversee highly sensitive investigations involving espionage and terrorism.

Administration officials insisted that Bush has the power to conduct warrantless surveillance under the Constitution's war powers provision. They argued that Congress also gave Bush the power when it authorized the use of military force against terrorists in a resolution adopted within days of the Sept. 11 attacks.