President Bush's chief political adviser, Karl Rove, is to testify again before a grand jury investigating a leak of the identity of a covert CIA operative.

Mr. Rove has already testified at least three times. The grand jury normally meets on Fridays, but it is unclear when Mr. Rove will testify again. His lawyer says the prosecutor has not yet made any decisions on filing charges.

New York Times reporter Judith Miller, who had initially been jailed for refusing to cooperate with the investigation, testified last week. The New York Times identified her source as Vice President Dick Cheney's chief of staff, Lewis Libby.

Investigators want to know who leaked the identity of covert CIA operative Valerie Plame. She is the wife of former Ambassador Joseph Wilson, who had accused the Bush administration of manipulating pre-war intelligence on Iraq.

Ambassador Wilson has charged members of the Bush administration leaked his wife's identity in retaliation for his criticism.

It is a federal crime to knowingly reveal a covert agent's identity.

Some information for this report provided by Bloomberg and Reuters.