The White House has strongly denied President George Bush's chief political strategist was involved in revealing the identity of a CIA woman whose husband questioned claims that Iraq had tried to buy uranium in Africa.
A Democratic senator has asked the Justice Department to appoint a special investigator to investigate the matter.
The naming of the intelligence officer's identity by syndicated columnist Mr Robert Novak came shortly after her husband, former ambassador Mr Joseph C. Wilson, undermined Mr Bush's claim that Iraq had tried to buy uranium in Africa.
Mr Wilson has publicly blamed Mr Karl Rove, Mr Bush's top political adviser, for the leak, although Mr Wilson did say he did not know whether Mr Rove personally was the source of Mr Novak's information, only that he thought Mr Rove had "condoned it."
A senior administration official said FBI officials were trying to determine whether there was a violation of the law and, if so, whether a full-blown criminal investigation was warranted.
Sen Charles Schumer said the matter should be investigated from someone outside the Bush administration.
On Sunday, national security adviser Ms Condoleezza Rice and Secretary of State Colin Powell both said they were unaware of any White House involvement in the matter. Mr Scott McClellan, the White House spokesman, reiterated the White House position and pledged co-operation.
The row began in January when Bush said in his State of the Union address that British intelligence officials had learned that Iraq had tried to purchase yellowcake uranium in Africa.
In an opinion piece published in July by The New York Times, Mr Wilson said he told the CIA long before President Bush's address that the British reports were suspect and the administration has since said the assertion should not have been in Mr Bush's speech.
A week after Wilson went public with his criticism Mr Novak, quoting anonymous government sources, said Mr Wilson's wife was a CIA operative working on the issue of weapons of mass destruction.
The Washington Posthas quoted an unidentified senior administration official as saying two top White House officials called at least a half-dozen journalists and revealed the identity and occupation of Mr Wilson's wife.