The US Department of Justice has launched a formal investigation into allegations that White House staff illegally blew the cover of a CIA agent.
Bush: "I want to know the truth"
Valerie Plame was named as a CIA agent by journalist Robert Novak after her husband, former diplomat Joseph Wilson, accused the Bush administration of exaggerating the case for war in Iraq.
It is alleged that two members of the White House staff leaked her name to Novak as an act of revenge.
Correspondents say it is extremely rare for the Department of Justice to conduct a full investigation into the alleged leaking of classified information.
President George W Bush said he welcomed the investigation and urged anyone with information to come forward.
"I want to know the truth," he said, adding that he did not know of anyone in his administration who leaked classified information.
Earlier, US Attorney General John Ashcroft told reporters: "The prosecutors and agents who are and will be handling this investigation are career professionals with extensive experience in handling matters involving sensitive national security information".
He refused to answer questions about the nature of the investigation.
White House staff were notified of the development in an e-mail on Tuesday after the Justice Department decided late on Monday to
move from a preliminary investigation to a full-blown inquiry.
Sent by White House counsel Alberto Gonzales, the e-mail directed all staff to preserve any materials that might be of relevance to the investigation.
These are likely to include telephone logs, e-mails, notes and other documents.
"The president has directed the White House to co-operate fully with this investigation," White House spokesman Scott McClellan told reporters.
"The president wants to get to the bottom of this."
Earlier, Mr McClellan strongly denied suggestions that Mr Bush's senior political adviser, Karl Rove, had approved the leak.
He added that there was nothing to suggest that any member of the White House staff was responsible.
Before news of the stepped-up inquiry broke, prominent members of the opposition Democratic Party had called for a special investigator to oversee an independent inquiry.
Ahead of the war in Iraq, Mr Wilson was sent by the CIA to the West African state of Niger in order to investigate claims that Iraq had tried to buy nuclear material there.
His report concluded that there was no evidence for the claims.
No evidence of weapons of mass destruction have been found in Iraq
Despite this, Mr Bush referred to them in his State of the Union address in January.
In a subsequent article in the New York Times, Mr Wilson questioned why his report had been ignored.
The White House later admitted it had been a mistake to include the claim in the address.
A report in the Washington Post newspaper has suggested that White House officials blew the cover of Mr Wilson's wife in order to discredit him, by suggesting he had been given the Niger mission only at her urging.
A leak of classified information is a federal crime punishable by up to 10 years in prison.