After nearly three months behind bars, a New York Times reporter, Judith Miller, was freed yesterday afternoon after agreeing to testify before a federal grand jury in an ongoing investigation into the alleged leak of a CIA operative’s identity.

Ms. Miller, who went to jail in July for refusing to testify in the probe, said she changed her stance after receiving assurances from a source she had spoken to confidentially that their discussions could be disclosed to prosecutors.

Yesterday’s developments came as Ms. Miller faced increased pressure to testify. The grand jury’s term is to expire next month and both the prosecutor and the judge in the case indicated that Ms. Miller could face criminal charges if she did not appear before the panel.

“I am leaving jail today because my source has now voluntarily and personally released me from my promise of confidentiality regarding our conversations,” the veteran reporter said in a written statement released by the Times. She said she will appear Friday before the grand jury that is investigating how the name of a CIA operative, Valerie Plame, appeared in the press in July 2003. The prosecutor, Patrick Fitzgerald, has said he is also examining whether Ms. Plame’s identity was leaked in an effort to strike back at her husband, Joseph Wilson, a vocal critic of President Bush.

The statement from the Times did not name Ms. Miller’s source, but the newspaper’s Web site identified him as Vice President Cheney’s chief of staff, I. Lewis Libby. Ms. Miller’s decision followed a phone conversation with Mr. Libby that was monitored by attorneys for both parties, the Times said.

Several attorneys said such precautions were wise because, if Ms. Miller had continued to defy the order to testify, a prosecutor could view the contact with Mr. Libby as obstruction of justice. “Where witnesses are talking about testimony, you want to be very careful about the prosecutor getting the wrong idea,” a lawyer involved in the case said, speaking on condition of anonymity.

That Ms. Miller’s incarceration stemmed from discussions she had with Mr. Libby is puzzling, lawyers tracking the inquiry said, because last year other journalists, including Matthew Cooper of Time magazine and Timothy Russert of NBC News, accepted Mr. Libby’s waiver of confidentiality and testified about their contacts with him.

Mr. Libby’s attorney, Joseph Tate, did not respond to a message left at his home last night. He said last year that he confirmed to lawyers for Mr. Russert and others that Mr. Libby viewed a generic written waiver he signed at the request of prosecutors as releasing the journalists from any duty of confidentiality.

The Time reporter, Mr. Cooper, avoided jail in July by testifying about a conversation he had with the president’s top political adviser, Karl Rove. That reprieve followed a similar turn of events in which Mr. Rove’s attorney affirmed that a waiver the White House aide signed long ago applied to Mr. Cooper.