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A special prosecutor has begun presenting evidence to a federal grand jury about the improper disclosure of an undercover CIA officer’s identity and has advised several people who have been employed at the White House that they could be summoned to testify, senior officials said.

The prosecutor, U.S. Atty. Patrick Fitzgerald of Chicago, began submitting evidence last week to a grand jury at the federal courthouse in Washington. It was not immediately clear who faced subpoenas.

At least some White House employees have been asked to meet informally with the prosecutor, possibly in an effort to obtain voluntary admissions of wrongdoing in exchange for offers of immunity, the officials said Friday. The list includes some relatively high-level staff members, the officials said.

Fitzgerald’s decision to use a criminal grand jury, first reported by Time magazine on its Web site, was not unexpected. But it did ratchet up the seriousness of the criminal inquiry, raising a possibility of courthouse appearances by Bush administration officials who would be compelled to testify under oath.

Law-enforcement officials said few details were known about the precise status of the case because Fitzgerald had operated in secrecy, keeping his prosecutor’s post in Chicago. At times he has refused to tell even his associates when he would arrive in Washington, for fear of inadvertently disclosing his intentions in the leak case.

Fitzgerald’s use of the grand jury could prove awkward for the White House, which has sought to control the damage over accusations that unidentified administration officials leaked the name of the CIA operative, Valerie Plame, in retaliation for her husband’s public criticism of President Bush’s policies on Iraq last summer.

Last week, a group of former intelligence officers pressed congressional leaders to open a separate review into the case because they were concerned that the Justice Department was not moving quickly enough.