WASHINGTON — A CIA employee who gave regular intelligence briefings to Vice President Cheney and his chief of staff, I. Lewis Libby Jr., testified yesterday that shortly after the leak of a CIA operative’s identity, the pair was given a stark warning that the leak could lead to the deaths of people who aided American intelligence gathering abroad.

“I thought there was a very grave danger to leaking the name of a CIA officer,” the briefer from Langley, Craig Schmall, said he told Messrs. Cheney and Libby during a morning session at the vice president’s residence. “Foreign intelligence services where she served now have the opportunity to investigate everyone whom she had come in contact with. They could be arrested, tortured, or killed.”

The testimony came in federal court yesterday at Mr. Libby’s trial on criminal charges that he obstructed an investigation to the leak to the press of the identity of the CIA operative, Valerie Plame. Since Mr. Schmall said his warning followed the leak, it did not suggest that Mr. Libby was given advance notice of the possible consequences of confirming Ms. Plame’s identity to reporters, as prosecutors contend Mr. Libby did.

However, the talk of the possibility of people being killed as a result of the leak could have an impact on the jury, and it offers another possible motive for why Mr. Libby allegedly scrambled to concoct a story about having thought he learned of Ms. Plame’s identity in the first instance from journalists, and not his colleagues in government.

Evidence put before the jury yesterday suggested that Mr. Schmall was one of those officials who discussed Ms. Plame with Mr. Libby before Robert Novak disclosed her identity in a syndicated column published on July 14, 2003.

In 2002, the CIA sent a former ambassador who is Ms. Plame’s husband, Joseph Wilson IV, to Africa to investigate reports that Iraq had sought to buy uranium there. Mr. Schmall’s notes of a briefing he gave Mr. Libby on June 14, 2003, reflect a discussion about the couple and the trip. Two days before that briefing, the Washington Post published an article that did not name Ms. Plame or her husband, but mentioned the trip and said it took place at the instigation of the vice president’s office.

“Why was the ambassador told this was a VP office question? Joe Wilson Valerie Wilson,” Mr. Schmall’s notation reads. Despite his handwriting on a table of contents for Mr. Libby’s intelligence briefing that day, the CIA briefer said he had “no recall” of discussing the subject of the Wilsons with Mr. Libby on that occasion.

Attorneys for Mr. Libby scored significant points by winning concessions from all three of yesterday’s prosecution witnesses that their memories of events key to the case had changed over time.

It is a time-honored defense tactic to question witnesses about inconsistencies between their courtroom testimony and prior statements they may have given to investigators or a grand jury.

However, the changes in recollection take on more weight in Mr. Libby’s case because part of his defense is that any misstatements he may have made to the FBI or the grand jury were simple mistakes brought on by faulty memory and overwork.

Judge Reggie Walton denied a defense request to call a scientific expert on memory lapses, so the defense team is intent on demonstrating the same point by highlighting the less-than-perfect recollection of each witness called by the prosecution.

On the receiving end of the most intense memory-related questioning yesterday was a former CIA official who oversaw the agency’s Iraq-related work and often attended meetings with Mr. Libby at the White House, Robert Grenier.

Prosecutors called Mr. Grenier to testify that, four days before Ms. Plame’s name was published, he told Mr. Libby during a telephone conversation that Mr. Wilson’s wife worked at the CIA. Mr. Grenier said the conversation was a response to questions Mr. Libby had posed about how Mr. Wilson’s trip was arranged and why reporters were claiming that it took place at Mr. Cheney’s behest.

“I recall having briefly felt guilty that I said too much,” Mr. Grenier said. Asked what he felt bad about, the ex-officer said, “Specifically, having noted that Mr. Wilson’s wife worked in the CIA. That’s in effect revealing the identity of an agency officer.”

While that account could undermine Mr. Libby’s claim to the FBI and the grand jury that he thought he learned about Ms. Plame from a journalist, the defense pointed out that Mr. Grenier did not relate that in his initial discussions with investigators or the grand jury.

“Your memory grew? You had a growing conviction?” a defense lawyer, Theodore Wells Jr., asked skeptically.

“I was saying what I believe to be true at the time and subsequently,” Mr. Grenier responded.

“Do you find that your memory gets better the more time away from an event you are?” Mr. Wells asked.

“It depends,” Mr. Grenier replied.

The testimony from the 27-year CIA veteran also provided ample evidence of the tension and mistrust between the agency and Mr. Cheney’s office in 2003 as questions loomed about why stocks of weapons of mass destruction had not been found in Iraq.

Mr. Grenier said he had a “strong suspicion” that White House officials leaked information to the Post as part of “an attempt to place blame for not having warned” the White House about Mr. Wilson’s conclusion that Iraq did not attempt to procure uranium from Niger.

The CIA official said he never read the details of Mr. Wilson’s findings but did not see how they could have been definitive.

In a lighter moment, Mr. Schmall said the reason he remembered his June 14, 2003, briefing was that Mr. Libby was “excited” about a meeting the White House aide had that morning with a celebrity couple, Tom Cruise and Penelope Cruz. “Tom Cruise was there to talk about how Germany treats Scientologists,” the CIA briefer said to tittering in the courtroom.

The trial was suspended for about 10 minutes as the high-level legal talent scoured the witness box and counsel tables looking for Mr. Grenier’s eyeglasses, which went missing as he was on the stand. Mr. Wells joked that his client might be charged with stealing the spectacles.

Mr. Fitzgerald, who is known as a dogged investigator, said there were limits to what he would do to find Mr. Grenier’s eyewear. “I ain’t patting him down,” the prosecutor said. “We’re hands on, but we ain’t that hands on.”