FBI agents will question Washington journalists and administration officials about claims that Mr Rove and others in the White House deliberately blew the cover of Valerie Plame, a CIA expert on weapons of mass destruction.

Ms Plame is the wife of Joe Wilson, a former US ambassador who in July accused the White House of misleading the nation over claims of Iraq's attempts to buy uranium in Africa. In a New York Times commentary he said he had been sent to Niger to check such claims in 2002 and found them to be baseless.

A few days after Mr Wilson went public with his allegations, a conservative columnist, Robert Novak, wrote that he had been told by "two senior administration officials" that Mr Wilson had been sent on the Niger mission at his wife's suggestion.

Reporters at Time magazine and NBC News and a handful of others were also tipped off about Ms Plame, and Democrats claim the source in each case was Mr Rove. According to some accounts, Mr Rove did not mention Ms Plame by name but referred to "Wilson's wife" being a CIA employee.

At a public meeting in August, Mr Wilson, a staunch Democrat himself, said: "It's of keen interest to me to see whether or not we can get Karl Rove frogmarched out of the White House in handcuffs. And trust me, when I use that name, I measure my words."