New York Times journalist Judith Miller has testified before a grand jury looking into whether White House sources leaked the name of a CIA agent.
Judith Miller said she believed in protecting sources
Miller - who spent 85 days in jail - said she agreed to testify only after formal authorisation from her source.
Miller has not named her source but her newspaper says he is understood to be Vice-President Dick Cheney's chief of staff, I Lewis "Scooter" Libby.
Mr Libby has denied leaking CIA agent Valerie Plame's name in 2003.
Ms Plame's name was leaked days after her husband - former ambassador Joseph Wilson - criticised the way intelligence on Iraq had been used to justify the US invasion.
The agent's name was made public by columnist Robert Novak but Miller had also done research into related matters and investigators wanted to know the identity of her source.
She was jailed in July because she refused to name her source. She was released on Thursday, when it was announced that Mr Libby was happy for accounts of conversations they had had to be given to the inquiry.
Mr Libby's lawyer Joseph Tate said "Scooter did not know the name until he saw it in the Novak article".
The disclosure of a CIA agent's name can be a federal offence.
If the inquiry finds that there was a deliberate leak, criminal charges might follow.
That would be a huge blow to the Bush administration, says the BBC's Justin Webb in Washington.
It is equally possible that no criminal charges will be filed and this affair will be a political embarrassment to the White House, nothing more, our correspondent says.
Special Counsel Patrick Fitzgerald, who began an inquiry into the whole affair, wanted to jail both Miller and another reporter, Matthew Cooper of Time magazine, over their conversations with White House sources.
In July, Miller and Cooper had refused to co-operate with the investigation, saying they should not have to reveal their sources because of press freedoms guaranteed in the US Constitution.
US journalists have rallied in support of Miller and Cooper
That defence was overruled by a court in Washington.
Cooper subsequently changed his mind, and agreed to testify, but Miller refused to do so, and was jailed.
After her testimony on Friday, she said: "I served 85 days in jail because of my belief in the importance of upholding the confidential relationship journalists have with their sources."
"Believe me, I did not want to be in jail. But I would have stayed even longer."
Mr Libby's lawyer says he has never had a problem with Judith Miller testifying at the grand jury.
Miller and Mr Libby met and talked by telephone in July 2003, the New York Times says.
The leak of Ms Plame's name was not made to Cooper or Miller, but they came to the attention of the prosecutor because of their inquiries.
The White House has consistently maintained that its staff were not responsible for the leak of Ms Plame's identity.