The chairman, Thomas H. Kean, the former Republican governor of New Jersey, also said in a Times interview that he believed the bipartisan 10-member commission would soon be forced to issue subpoenas to other executive branch agencies because of continuing delays by the Bush administration in providing documents and other evidence needed by the panel.
Sen. Chuck Hagel, R-Neb., said in a broadcast interview on Sunday that it would be in the administration's interest to release documents the commission has requested.
"Americans and our allies across the globe must have confidence in our leadership," said Hagel, a senior member of the Senate Foreign Relations Committee who has frequently criticized Bush's execution of the campaign against terrorists. "They must trust our processes. And that certainly includes our intelligence community's results."
The 10-member, bipartisan commission has until May 27 to submit a report that also will deal with law enforcement, diplomacy, immigration, commercial aviation and the flow of assets to terror organizations.
Sen. Joseph Lieberman, D-Conn., co-author of the legislation that created the independent commission, called on the White House to turn the documents over immediately.
"If they continue to refuse, I will urge the independent commission to take the administration to court," said Lieberman, who is running for president. "And if the administration tries to run out the clock, (Arizona Republican Sen.) John McCain and I will go to the floor of the Senate to extend the life of the commission."
"Any document that has to do with this investigation cannot be beyond our reach," Kean told the Times in his first explicit public warning to the White House that it risked a subpoena and a politically damaging courtroom showdown with the commission over access to the documents, including Oval Office intelligence reports that reached President Bush's desk in the weeks before the Sept. 11 attacks.
"I will not stand for it," Kean said in the interview in his Madison, N.J. offices at Drew University, where he has been president since 1990. "That means that we will use every tool at our command to get hold of every document."
He said to the Times that, while he had not directly threatened a subpoena in his recent conversations with the White House legal counsel, Alberto R. Gonzales, "it's always on the table, because they know that Congress in their wisdom gave us the power to subpoena, to use it if necessary."
A White House spokeswoman, Ashley Snee, said the White House believed it was being fully cooperative with the commission, which is known formally as the National Commission on Terrorist Attacks Upon the United States. She said that it hoped to meet all of the panel's demands for documents, the Times reports.
Kean suggested to the Times that he understood the concerns of the White House about the sensitivity of the documents at issue, saying that they were the sort of Oval Office intelligence reports that were so sensitive and highly classified that they had never been provided to Congress or to other outside investigators.
"These are documents that only two or three people would normally have access to," he said. "To make those available to an outside group is something that no other president has done in our history.
"But I've argued very strongly with the White House that we are unique, that we are not the Congress, that these arguments about presidential privilege do not apply in the case of our commission," Kean added.
"Anything that has to do with 9/11, we have to see it — anything. There are a lot of theories about 9/11, and as long as there is any document out there that bears on any of those theories, we're going to leave questions unanswered. And we cannot leave questions unanswered."
While Kean said he was barred by an agreement with the White House from describing the Oval Office documents at issue in any detail — he said the White House was "quite nervous" about any public hint at their contents — other commission officials said they included the detailed daily intelligence reports that were provided to Mr. Bush in the weeks leading up to Sept. 11. The reports are known within the White House as the Presidential Daily Briefing.
Despite the threat of a subpoena and his warning of the possibility of a court battle over the documents, Kean said he maintained a good relationship with Gonzales and others at the White House, and that he was still hopeful the White House would produce all of the classified material demanded by the panel without a subpoena.
"We've been very successful in getting a lot of materials that I don't think anybody has ever seen before," he said of his earlier dealings with the White House. "Within the legal constraints that they seem to have, they've been fully cooperative. But we're not going to be satisfied until we get every document that we need."
Last year, the White House confirmed news reports that President Bush received a written intelligence report in August 2001, the month before the attacks, that Al Qaeda might try to hijack American passenger planes.
Snee, the White House spokeswoman, said, "The president has stated a clear policy of support for the commission's work and, at the direction of the president, the executive branch has dedicated tremendous resources to support the commission, including providing over two million pages of documents."
After months of stating that it believed subpoenas to the executive branch would not be necessary, the commission voted unanimously this month to issue its first subpoena to the Federal Aviation Administration after determining that the F.A.A. had withheld dozens of boxes of documents involving the Sept. 11 attacks.