Democrats demanded the appointment of a special outside counsel, but Mr. Bush said he was "absolutely confident" that the Justice Department can do a good job.
"I don't know of anyone in my administration who has leaked," Mr. Bush told reporters in Chicago. But, he added, "If somebody did leak classified information, I'd like to know it, and we'll take the appropriate action. And this investigation is a good thing."
The president added, "There's too much leaking in Washington. That's just the way it is. We've had leaks from the executive branch and leaks from the legislative branch. I want to know who the leakers are."
Most White House employees discovered the probe was under way when they turned on their computers and found an e-mail shortly before 9 a.m. Tuesday that said: PLEASE READ: Important Message From Counsel's Office.
"You must preserve all materials that might in any way be related to the department's investigation," counsel Alberto Gonzales ordered. Officials indicated that would include telephone logs, e-mails, notes and other documents.
The investigation is aimed at finding who leaked the name of the CIA operative, possibly in an attempt to punish the officer's husband, former Ambassador Joseph C. Wilson, who had accused the administration of manipulating intelligence to exaggerate the threat from Iraq.
Democrats contend the Bush Justice Department cannot fairly investigate the Bush White House and called for a special counsel to look into the matter.
Four Senate Democrats, including Minority Leader Tom Daschle, D-S.D., urged Attorney General John Ashcroft to appoint a person of "unquestioned independence and impartiality."
"We do not believe that this investigation of senior Bush administration officials ... can be conducted by the Justice Department because of the obvious and inherent conflicts of interests involved," said the letter, also signed by Sens. Carl Levin, D-Mich., Joseph Biden, D-Del. and Jay Rockefeller, D-W.Va.
Ashcroft declined a direct response to their calls for a special counsel, saying Tuesday that "because of an ongoing investigation of criminal violations, I will not be making any further comment at this time."
Later, the Justice Department said "no options are closed" when asked if a special prosecutor has been ruled out. "We are not closing any legal doors," spokesman Mark Corallo said.
Law enforcement sources say regardless of who's in charge, the chances of this investigation going anywhere are next to none, reports CBS News Chief White House Correspondent John Roberts. A year after it happened, the FBI is still investigating a leak of classified information about Osama Bin Laden from the Senate Intelligence Committee.
"It's always difficult to prosecute leak cases because the person who leaked it almost never admits that he did it, and absent a confession it's almost impossible to find out who did it," Jeffrey Smith, former Justice Department general counsel, tells CBS.
On Monday, administration officials vigorously denied anyone at the White House leaked the CIA officer's identity.
"There's been nothing, absolutely nothing brought to our attention to suggest any White House involvement, and that includes the vice president's office as well," McClellan said.
In particular, McClellan said it was "ridiculous'' to suggest that Karl Rove, the president's top political operative, was involved, as Wilson once charged. "He wasn't involved," McClellan said of Rove. "The president knows he wasn't involved. ... It's simply not true."
Wilson backtracked Monday, saying Rove did not make the calls but either condoned or did not attempt to step the leak.
The Houston Chronicle reported Tuesday that in 1992, Rove was dismissed as a consultant for the Bush-Quayle Texas campaign after he was suspected of leaking a story to Bob Novak and then-partner Roland Evans about problems in the campaign.
At the time, Rove denied he was the source of the leak, the Chronicle says.
Novak wrote that he got the CIA agent's name from "two senior administration officials." He said the CIA confirmed her role and "asked me not to use her name, but never indicated it would endanger her or anybody else."
The Washington Post reported that a White House official called six reporters with the same agent's name, but Novak was the only one to release it.