President Bush's top political adviser, Karl Rove, testified for a fourth time Friday before a grand jury investigating the leaking of the identity of a covert CIA operative.

Karl Rove made no comment after several hours of testimony before the grand jury probing White House involvement in the case of Valerie Plame, a covert CIA operative married to former Ambassador Joseph Wilson.

Ambassador Wilson was a critic of pre-war intelligence on Iraq, and special prosecutor Patrick Fitzgerald is investigating whether top Bush aides leaked Ms. Plame's identity to journalists as a way of retaliating against her husband.

It is a federal crime to knowingly reveal the identity of a covert intelligence agent.

Grand jury testimony is held in secret, and prosecutor Fitzgerald has not said why Mr. Rove was brought back for a fourth appearance.

Prosecutors have told Mr. Rove they cannot guarantee that he will not be indicted on criminal charges in connection with the leak probe. Mr. Rove has denied leaking Ms. Plame's identity.

U.S. Attorney General Alberto Gonzales was asked about the case this week on NBC television.

"This prosecutor may have new information that may contradict prior testimony," said Alberto Gonzales.

Some supporters of the president have expressed concern that Mr. Rove and Lewis Libby, Vice President Dick Cheney's chief-of-staff, could face criminal indictments in connection with the Plame investigation sometime in the next few weeks.

There has been no comment from prosecutors on whether criminal indictments may be coming.

At the White House Friday, spokesman Scott McClellan denied suggestions that the Bush administration has been distracted by the CIA leak investigation.

"We have got a lot of big challenges facing this country, and the president is focused on addressing those challenges," said Scott McClellan. "That is where he is keeping his focus."

The president has been asked about the case several times in recent weeks, but says it would be inappropriate to comment while the investigation is still under way.

"I am not going to talk about the case," said President Bush. "It is under review. So, I am not going to talk about it."

Some opposition Democrats have previously called on the president to fire Mr. Rove, who has long been Mr. Bush's top political adviser.

The focus on the leak case comes at a difficult time for the Bush White House. The president's public approval ratings are at an all-time low because of concerns about Iraq, domestic fuel prices and the government response to Hurricane Katrina.