Government prosecutors have interviewed US President George W Bush as part of their investigation into how an undercover agent's cover was blown.
Bush welcomed the inquiry, saying he wanted "to know the truth"
US Attorney Patrick J Fitzgerald questioned the president in the Oval Office for 70 minutes.
The investigation aims to find out how the name of CIA agent Valerie Plame was leaked to a journalist.
Ms Plame's husband, former ambassador Joseph Wilson, claims his criticism of the Iraq war prompted the leak.
A spokesman for the White House said the president had appointed a lawyer to act on his behalf during the investigation.
"The leaking of classified information is a very serious matter," the spokesman said.
President Bush has said he wants his administration to co-operate with the investigation, he added.
"[Mr Bush] was pleased to do his part [on Thursday]," the spokesman said.
Since January, 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.
Disclosing a covert agent's name is a criminal offence in the US, punishable by up to 10 years in prison.
The investigations centre on the circumstances surrounding the release of Ms Plame's name last July.
Before the war in Iraq, her husband, the former US charge d'affaires in Baghdad, was sent by the CIA to the West African state of Niger 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.
It was claimed Iraq had tried to buy nuclear material in Niger
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, Ms 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.