Grand Jury questions White House aides over outing of undercover CIA agent Valerie Plame

The controversy over the outing of an undercover CIA agent continued to threaten President George Bush yesterday when it was revealed that some of his most senior officials had been questioned by FBI investigators looking into the leak. Senior officials from the Vice-President's office have also testified as part of the criminal inquiry.

Scott McClellan, a White House spokesman, and Karl Rove, Mr Bush's chief political adviser, are among the officials who have been questioned. Mr McClellan said that he gave evidence before a grand jury last week - a process in which witnesses are questioned before a jury panel which decides whether there is sufficient evidence to proceed. Mr Rove, considered one of the most powerful figures in the Bush administration, was also questioned recently.

Mr McClellan said: "[I was] doing my part to co-operate as the President asked us all to do."

The FBI inquiry relates to the leaking of the identity of the CIA operative Valerie Plame, the wife of a former US ambassador, Joseph Wilson. It is alleged that the leak was an act of retaliation against the diplomat, who had exposed one of the Bush administration's false claims about Iraq's efforts to produce nuclear weapons.

Her identity as a weapons expert was leaked to a right-wing newspaper columnist after Mr Wilson revealed that a fact-finding trip he carried out to Niger in 2002 at the behest of the government proved that claims made by the President that Iraq had been trying to buy uranium from the West African nation were false. The White House was forced to admit that the claim should not have been included in the President's 2003 State of the Union address.

The affair has the potential to be hugely damaging to Mr Bush as he campaigns for re-election. Having repeatedly argued that he is a president who has stood up for national security, it would be greatly embarrassing to have one of his senior officials accused of ruining Ms Plame's career and potentially threatening the safety of some of those she had worked with overseas.

The leaking of the name of an undercover officer is a criminal offence and, under sustained pressure from both Democrats and Republicans, the administration appointed a special prosecutor in December to lead the FBI's inquiry into the incident.

Reports published yesterday said that agents had gathered scores of e-mails and phone records and interviewed numerous senior officials including the Vice-President Dick Cheney's former adviser Mary Matalin, Adam Levine, a former White House press official, Dan Bartlett, the White House communications director, Catherine Martin, an aide to Mr Cheney and Ari Fleischer, the former White House spokesman. Some of the meetings were said to be "tense and combative".

The investigators have also questioned Mr Cheney's chief of staff, Lewis "Scooter" Libby, and during their interviews with other officials the FBI has repeatedly referred to "copious" notes compiled by him. It is believed that the FBI officers have also interviewed John Hannah, another aide to Mr Cheney, and may be offering an incentive to him to reveal what he knows about other officials. Some reports have suggested that Mr Libby could be charged.

Larry Johnson, a former CIA operative and a long-time friend of Ms Plame, said he had been told that a number of other advisers to the White House were also at the centre of the FBI inquiry. "This is really beginning to heat up," he said.

Ms Plame's identity was revealed last summer by the veteran conservative columnist Robert Novak. It is believed that at least six other journalists in Washington were contacted by two White House officials who disclosed the operative's details. Mr Novak - who has refused to reveal his sources - and the other journalists could find themselves subpoenaed to appear before the inquiry.

The inquiry into the leak of Ms Plame's identity is being headed by Patrick Fitzgerald, the US attorney in Chicago. A spokesman for Mr Fitzgerald refused to comment on those questioned, saying he was not legally permitted to talk about them. Mr Cheney's office has also declined to comment.

It was reported yesterday by the The Washington Post that a parallel FBI inquiry is investigating the forgery of documents suggesting that Iraq was seeking to buy the uranium "yellowcake" from Niger.

That inquiry, which is being carried out by FBI counter-intelligence agents, was launched last spring after experts at the International Atomic Energy Agency (IAEA) revealed that the documents obtained by the US were fake.