Press secretary Scott McClellan and Adam Levine, who formerly worked in the press office, testified before the panel on Friday. A deputy in McClellan's office, Claire Buchan, said she appeared before the panel on Jan. 30.
"I was pleased to cooperate," said Buchan, who declined to reveal any details of her testimony.
One person close to the investigation said that Levine was questioned mainly about White House press office procedures.
Mary Matalin, a Republican consultant who headed Vice President Dick Cheney's communications office until December, confirmed that she testified on Jan. 21.
A number of other current or former officials have been interviewed by the FBI, including top Bush political adviser Karl Rove, communications chief Dan Bartlett, former spokesman Ari Fleischer and Cheney spokeswoman Cathie Martin, said two sources who spoke on condition of anonymity.
The Washington Post reports the FBI has also talked to Cheney chief of staff I. Lewis "Scooter" Libby, along with Levine, Matalin and McClellan.
McClellan declined to provide details on how many other Bush administration officials had testified or been interviewed. "We fully expect everyone in the White House to do their part to cooperate," he said.
The Post reports the FBI has conducted aggressive questioning covering White House emails that were turned over to the investigators, cell phone calls, phone logs and even specific conversations with reporters.
The investigation concerns a July 14 article by columnist Robert Novak, in which he named the wife of former Ambassador Joseph Wilson as a CIA operative. Novak quoted two "senior administration officials" in his article.
Wilson had a few weeks earlier emerged to report that he had personally investigated the allegation that Iraq tried to buy uranium from Niger, and found no substantiation for it.
But President Bush made the Niger allegation in the Jan. 28, 2003 State of the Union speech.
After Wilson came forward in July, the White House retracted that allegation. Wilson now is working as a foreign policy adviser to Democratic presidential candidate John Kerry and has made several campaign appearances for the Massachusetts senator.
Whoever leaked the name could be charged with a felony under the Intelligence Identities Protection Act, which makes it a crime to knowingly and intentionally reveal the name of a covert operative
After a request from the CIA, the Justice Department launched an investigation into the leak on Sept. 30.
Attorney General John Ashcroft removed himself in December from the inquiry to avoid the appearance of a conflict of interest. Democrats had called on Ashcroft to step aside from the investigation from the outset. U.S. Attorney Patrick J. Fitzgerald of Chicago, a career prosecutor, is leading the probe.
The Post reports the FBI soon may seek to pressure individual journalists into revealing their sources.
Investigators earlier tried to encourage journalists to talk by asking White House staff to sign waivers freeing reporters of any promises of confidentiality. But according to The Post, most officials were advised by the lawyers not to sign, and did not.
Meanwhile, The Post reports, a separate probe into forged documents that the United States thought were evidence of the Niger deal is "at a critical stage," an official said.
In his State of the Union, Mr. Bush said: "The British government has learned that Saddam Hussein recently sought significant quantities of uranium from Africa."
That was only one of several occasions that same month where the administration made a similar claim. The Post reports the claim was repeated on at least six occasions around the time of the State of the Union speech.
Shortly after Mr. Bush's speech, the U.S. turned over to the International Atomic Energy Agency documents purporting to prove an Iraqi bid for uranium to Niger. Within days, the IAEA reported the papers were forgeries.
It was unclear how the allegation made it into the State of the Union speech, normally the president's biggest speech of the year and the subject of months of preparation.
The CIA, doubting the Africa link, tried to get British intelligence to omit the claim from its September 2002 dossier on Iraq's suspected weapons programs. The British went ahead with the claim anyway, and it became the basis for the president's speech.
The White House first insisted it was unaware before the Jan. 28 speech that there were problems with the intelligence underlying the claim.
But Stephen Hadley, No. 2 on the president's national security team, later disclosed that two CIA memos and a call from CIA Director George Tenet had persuaded him to take a similar passage out of a presidential speech in October — and that he should have done likewise when it turned up again in State of the Union drafts.
Hadley said he had forgotten about those objections by the time the State of the Union speech was being crafted.
The uranium claim has become the best known of the Bush administration's allegations against Iraq, which included charges that Iraq possessed biological and chemical weapons, was working toward nuclear arms, and had active links to terrorist groups like al Qaeda.
No weapons have been found.
Late last year, the president's Foreign Intelligence Advisory Board said a panel had concluded the White House made "no deliberate effort to fabricate" the claim that Iraq had tried to buy uranium in Africa.
Instead, the claim was written into the president's speech as part of an effort "to grab onto something affirmative" about Saddam Hussein's alleged nuclear program.