Independent Prosecutor Patrick Fitzgerald -- who has been investigating the leak of the name of CIA agent Valerie Plame -- had reported to Comey because Attorney General Albert Gonzales was recused from the case. As the case nears conclusion, Comey's replacement could be an important decision.
Comey had granted Fitzgerald virtual carte blanche to conduct the investigation.
Newsweek reports Associate Attorney General Robert McCallum is "likely" to fill in for Comey. McCallum is a close friend and "Skull and Bones" brother of President George W. Bush from their days as students at Yale.
Fitzgerald's office has called Bush's top political adviser, Karl Rove, to testify before a federal grand jury at least three times.