uments that were "highly material to our inquiry."

The bipartisan commission also warned that it was considering subpoenas for material from other executive branch agencies and that the resulting delays could force it to extend its investigation beyond May, when it is supposed to complete its work.

The possibility of an extension is worrying to the Bush administration, since it could mean the public release of a potentially embarrassing report in the heat of next year's presidential campaign.

In a statement, the 10-member commission said it learned within the last few days that "various tapes, statements, interview reports and agency self-assessments highly material to our inquiry inexplicably had not been included" in the materials from the aviation agency. "It is clear that the F.A.A.'s delay has significantly impeded the progress of our investigation," the statement said.

The statement offered no other details about the documents that are the subject of the subpoena, which the commission approved on Tuesday night in a private meeting.

Government officials with knowledge of the commission's work said the panel and its staff were particularly alarmed by the discovery that they had not been provided with detailed transcripts and other information about communications on Sept. 11 between the the F.A.A. and the North American Aerospace Defense Command, or Norad, the unit of the Pentagon that is responsible for defending American air space.

The panel has been studying whether the aviation agency, Norad and other agencies reacted too slowly on the morning of Sept. 11 after four passenger planes had been hijacked.

In a statement on Wednesday, the F.A.A. promised full cooperation with the commission and said it recognized that its procedures for gathering and distributing material for the investigation were "not satisfactory to the commission and its staff - no documents were ever knowingly withheld from the commission."

"We are surprised by the commission's decision to formally subpoena the agency's records, a step we regard as unnecessary," the agency said. "The agency will respond fully to the subpoena."

The commission's decision drew bipartisan expressions of concern in Congress, which created the panel - over the initial objections of the Bush administration - in hopes of an independent review of intelligence and law enforcement lapses that may have allowed the Sept. 11 terrorists to plan and carry out the attacks.

"I am deeply concerned to learn that the F.A.A. has apparently both misled and failed to adequately respond to the 9/11 commission's request for documents," said Senator John McCain, the Arizona Republican who was a primary sponsor of the bill creating the commission last year.

Senator Joseph I. Lieberman of Connecticut, the lead Democratic sponsor, said that the significance of the subpoena was clear: "This administration is standing in the way of a thorough and searching inquiry."

The bill creating the commission provided the panel with subpoena power, although its chairman, Thomas H. Kean, the former Republican governor of New Jersey, had repeatedly said that he was hesitant to use it, in part because of the delays that it might cause.

In July, Mr. Kean and the panel's deputy chairman, Lee H. Hamilton, a former Democratic member of the House from Indiana, warned that several agencies had been slow to meet requests for documents and other evidence. But in recent weeks, the commission has said that cooperation had markedly improved.

In its statement, the commission said it was assured by the aviation agency last month that it had provided the panel with all available information in its files pertinent to the panel's work. But the commission said that in recent days, in interviews with employees of the agency, the panel was told of other material that had not been turned over.

"Once this issue came to light -just in the past few days - the F.A.A. provided the commission with dozens of boxes and materials that its representatives now claim satisfy our request, and they pledged the F.A.A.'s full cooperation," the panel said. "This disturbing development at one agency has led the commission to re-examine its general policy of relying on document requests rather than subpoenas."

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