US national security adviser Condoleezza Rice will testify publicly on 8 April before the 11 September 2001 commission.
Rice testimony is likely to counter charges that President George Bush did not make terrorism an urgent priority before 9/11.
Responding to heavy political pressure from both Republicans and Democrats, the White House made an abrupt about-face on Tuesday and agreed to allow Rice to testify publicly and under oath after insisting she only speak to the panel privately.
A key area of questioning for Rice is expected to be claims by former US counterterrorism chief Richard Clarke that Bush ignored an urgent al-Qaida threat before the 9/11 attacks and was fixated on Iraq.
At the White House, which was battered by criticism at the refusal to let Rice testify, there was hope that the appearance by the well-spoken Rice would allow the administration to get the last word on Clarke and turn the page on the bad news of the past week.
Bush's re-election strategy rests a great deal on his performance in the "war on terrorism" and the White House is sensitive to any suggestion that he was not doing enough to try to prevent the attacks.
Secretary of State Colin Powell, in a visit to Berlin, told ZDF German television that the Bush administration "did as much as we could, knowing what we knew about the situation."
Colin Powell: "We did as much as
"We raised our threat levels. We warned our embassies. We
warned our people around the world. We made sure our military was safe and were not exposed... We did everything we could to protect ourselves," Powell said.
The White House fought against an impression left in an article by The Washington Post that Bush, Rice and others in the top echelon of power were more concerned about missile defense than terrorism in the months before 9/11.
The Post published excerpts of a speech that Rice was to deliver on the evening of 11 September 2001, that the newspaper said was designed to promote missile defense as the cornerstone of the Bush administration's national security.
"You're talking about one speech," said White House spokesman Scott McClellan. "I think you need to look at the actions and concrete steps that we were taking to confront the threat of terrorism."
"[Rice speech] is just the final cherry on the
pudding proving that what these people were concerned about was not al-Qaida or Usama bin Ladin but madmen with missiles."
A foreign policy analyst at the Brookings Institution
The White House would not reveal the entire text of the aborted speech, prompting a request from New York Democratic Senator Charles Schumer that it be released.
Ivo Daalder, a foreign policy analyst at the Brookings Institution who worked on Democratic President Bill Clinton's National Security Council, said the Bush administration would be hard-pressed to find any reference to al-Qaida and Usama bin Ladin by any top officials in the months before 11 September.
The Rice speech, he said, "is just the final cherry on the pudding proving that what these people were concerned about was not al-Qaida or Usama bin Ladin but madmen with missiles."
A new Los Angeles Times poll found 52% of Americans agreed with Clarke's charges that Bush failed to take the threat of terrorism seriously enough before 9/11.
Questions arose in Washington about contacts between the Bush administration and Republican commissioners as they prepared to grill Clarke about his charges last week.
People close to the commission said White House counsel Alberto Gonzales called commissioners Fred Fielding as well as James Thompson. The two commissioners went on to sharply
McClellan would not confirm the calls. He accused Rep Henry Waxman, ranking Democrat on the House Government Reform Committee, of trying to "politicise" the commission's deliberations by asking the White House to detail Gonzales'
conversations with the commissioners.