WASHINGTON — A former reporter for Time magazine testified that I. Lewis “Scooter” Libby Jr. provided confirmation to him that the wife of a Bush administration critic worked for the CIA, but only after the reporter first learned of her identity from President Bush’s chief political adviser, Karl Rove.

Testifying for the prosecution at Mr. Libby’s perjury trial here, the reporter, Matthew Cooper, told jurors that on July 11, 2003, Mr. Rove tipped him off to the CIA affiliation of the wife of a former American ambassador, Joseph Wilson IV, who days earlier had published an opinion piece critical of the administration’s justification of its invasion of Iraq.

When Mr. Cooper asked Mr. Libby about the tip the next day, the vice president’s chief of staff responded with words to the effect of, “Yeah, I’ve heard that, too,” Mr. Cooper testified. He said he later read that Mr. Wilson’s wife was Valerie Plame, information that was published in a column by Robert Novak on July 14, 2003.

Mr. Cooper bolstered the prosecution’s case by saying Mr. Libby never told him he learned about Mr. Wilson’s wife from other reporters, which is what he told a grand jury under oath. Mr. Libby has pleaded innocent to five counts of perjury, obstruction of justice, and making false statements.

But Mr. Cooper’s testimony that Mr. Rove first disclosed to him the CIA status of Mr. Wilson’s wife could play into an argument of the defense — that the White House sacrificed Mr. Libby, who resigned upon his indictment in 2005, to protect Mr. Rove, who remains a top aide to the president.

The testimony came as the defense spent hours scrutinizing the notes and reporting practices of Mr. Cooper and a former reporter for the New York Times, Judith Miller, in an effort to cast doubt on their recollections of conversations with Mr. Libby and others.

The defense also indicated late yesterday afternoon that it would subpoena the managing editor of the Times, Jill Abramson, to counter testimony by Ms. Miller, who left the paper in 2005. A lawyer for Mr. Libby, William Jeffress Jr., told Judge Reggie Walton that he expected the Times to fight the subpoena. A lawyer for the paper did not return a message last night.

Facing a second day of aggressive cross-examination, Ms. Miller acknowledged that she could not be “absolutely, absolutely certain” that Mr. Libby was the first official to tell her that Ms. Plame worked for the CIA. She escaped, however, without having to disclose any more confidential sources. Mr. Jeffress pressed her to identify other officials with whom she discussed Ms. Plame, but she insisted she could not remember their names.

When Mr. Jeffress changed the topic at one point, Ms. Miller smiled widely. She let out a sigh of relief as she walked out of the courtroom upon finishing her testimony. Ms. Miller spent 85 days in jail in 2005 after refusing to name her sources before a grand jury impaneled by the prosecutor in this case, Patrick Fitzgerald.

Mr. Jeffress dissected Mr. Cooper’s testimony as well, questioning how he could take Mr. Libby’s statement about Mr. Wilson’s wife as confirmation of her CIA status, knowing that the comment was off the record and never having asked Mr. Libby where he heard it. The lawyer displayed Mr. Cooper’s typewritten notes from the conversation, which were riddled with typographical errors and, according to Mr. Cooper, contained no reference to Mr. Wilson’s wife.

Yesterday’s proceeding also unveiled a bit of Washington gossip. The prosecution unsuccessfully sought to introduce as evidence a note written by Mr. Libby that quotes a former vice presidential aide, Mary Matalin, as calling Mr. Wilson “a snake.”

According to the note, Ms. Matalin advises that the administration take its rebuttal of Mr. Wilson to an NBC News journalist, Tim Russert, to counter a fellow NBC journalist, Chris Matthews, who played up the former diplomat’s criticism of the White House on his MSNBC show “Hardball.”

“He hates Chris,” Ms. Matalin said of Mr. Russert, according to the prosecution.