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Presidential adviser Karl Rove has agreed to testify again to a grand jury in the investigation into the leak of a covert CIA agent’s identity.

It is unclear why Rove has been asked to make another trip, his fourth, to the grand jury looking into who leaked the name of agent Valerie Plame.

It is a felony to knowingly leak the name of a covert CIA official, and the case involves some of the most powerful officials in the White House and some of the country’s most prominent journalists.

Rove’s attorney, Robert Luskin, said he told prosecutors in July–when Time magazine’s Matt Cooper told the grand jury about his conversations with Rove–that the key adviser to President Bush would be willing to cooperate in any way with the investigation.

After New York Times reporter Judith Miller testified before the grand jury last week, Luskin said, “I was advised … that [prosecutors] might like his further cooperation and the prosecutor affirmed that he has made no decision on whether to charge Karl,” Luskin said.

He declined to comment when asked whether there is an increased likelihood that his client would face charges. Luskin said Rove has not received a “target letter,” customarily sent to grand jury witnesses about to be indicted.

The Associated Press, citing as its source several people familiar with the investigation, reported that special counsel Patrick Fitzgerald, who is U.S. attorney in Chicago, sent a letter accepting Rove’s offer to testify but warning that prosecutors could no longer guarantee that Rove wouldn’t be indicted.

According to the U.S. attorney’s manual, prosecutors may not bring witnesses before a grand jury if there is a possibility of future criminal charges unless the witnesses are notified in advance that their testimony can be used against them in a later indictment.

Rove’s appearance before the grand jury would occur as the panel nears the end of its term under the guidance of Fitzgerald.

Among other avenues, Fitzgerald has been investigating whether White House officials, including Rove and Lewis “Scooter” Libby, Vice President Dick Cheney’s chief of staff, were fully forthcoming with investigators about their knowledge of how Plame’s name became public.

A spokesman for Fitzgerald said the prosecutor and his office would have no comment.