Expected indictments in the case could trigger an immediate shake-up at the White House, already on the defensive over plummeting poll figures, soaring gas prices, opposition to the Iraq war and the withdrawal of President George W. Bush's nominee for the US Supreme Court, Harriet Miers.
Fitzgerald has zeroed in on Lewis Libby, chief of staff for Vice President Dick Cheney, and Karl Rove, Bush's top political adviser. Other current and former administration officials may also face charges.
White House officials were anxiously awaiting the outcome since any indicted officials were expected to resign immediately. Bush was then likely to make a public statement.
Fitzgerald's spokesman, Randall Samborn, said no announcements were expected on Thursday, leaving any legal action for Friday, when the grand jury hearing the case is scheduled to meet for the last time. Fitzgerald appeared unlikely to seek an extension.
Fitzgerald spent the day huddled in Washington with his deputies as he made final preparations to wrap up the two-year leak investigation.
On Wednesday, he met for three hours with the grand jury, and spent 45 minutes behind closed doors with the chief US district judge, Thomas Hogan.
In a last-minute flurry of interviews, FBI agents this week canvassed Plame's neighbourhood to see if anyone knew about her covert work for the spy agency before she was outed in a July 2003 newspaper column by Robert Novak.
Fitzgerald met with Rove's attorney, Robert Luskin, and prosecutors conducted an 11th-hour interview with Adam Levine, a former official in the White House press office, about his conversations with Rove.
Plame's identity was leaked to the media after her diplomat husband, Joseph Wilson, accused the administration of twisting pre-war intelligence on Iraq. Wilson said it was done deliberately to erode his credibility.
The White House initially denied that Libby and Rove had anything to do with the leak, but reporters have singled them out as sources in grand jury testimony.