The US authorities have named a special counsel to lead an investigation into who leaked the identity of a CIA agent whose husband opposed the Iraq war.
Fitzgerald has led several high-profile investigations
Patrick Fitzgerald, the US Attorney in Chicago, will lead the probe, US Deputy Attorney General James Comey said.
Mr Comey also said Attorney General John Ashcroft had decided to withdraw from the politically sensitive case.
The inquiry is looking into claims that White House staff illegally blew the cover of the agent, Valerie Plame.
Disclosing a covert agent's name is a criminal offence in the US, punishable by up to 10 years in prison.
Making the announcement at the Justice Department, Mr Comey said Mr Fitzgerald "has the power and authority to make whatever prosecutorial judgement he needs" and would report to him personally.
Bush welcomed the inquiry, saying he wanted "to know the truth"
Mr Comey also said he agreed with Mr Ashcroft's decision to withdraw from the case.
"The attorney general, in an abundance of caution, believed that his recusal was appropriate, based on the totality of circumstances and the facts in evidence developed at this stage of the investigation," he said.
Democrats in the US Congress had demanded that Mr Ashcroft - who was appointed by President Bush - should step aside from the inquiry and make way for a more neutral prosecutor.
The Department of Justice launched a full investigation into the alleged leaking of classified information in September - which experts described as an extremely rare move.
Dozens of Bush administration officials have been interviewed, and the inquiry has extended beyond the White House to other government agencies, including the Defence and State departments.
Act of revenge?
The allegations centre on July's disclosure of Valerie Plame, wife of Joseph Wilson, the former US charge d'affaires in Baghdad.
Before 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.
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 week after Mr Wilson's article appeared, Valerie Plame was exposed by journalist Robert Novak, who said he based his report on two unidentified senior administration officials.
A report in the Washington Post newspaper later 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.