"Unfortunately NORAD has not complied with our long-standing document requests," said Commissioner Richard Ben-Veniste.
He said that after a series of field inquiries and interviews with NORAD personnel, commission staff had realized, "quite recently" that "the materials (NORAD) had previously provided were incomplete."
"Our staff will have to backtrack, re-interview people and duplicate effort," said Ben-Veniste. He said the failure would cause "significant delays" to the commission's work, and might mean the postponement of a hearing planned for January.
That hearing, like an earlier one in June, dealt with the immediate response of federal agencies and the military to the news on Sept. 11, 2001 that several aircraft had been hijacked and one of them flown into the World Trade Center.
Last month the commission issued a subpoena to the Federal Aviation Administration for material covering their response. Commission spokesman Al Felzenberg said the subpoena to NORAD -- the military entity responsible for the defense of the nation's skies -- covered similar issues.
At stake is the vexed question of whether a swifter response might have made it possible to shoot down the other hijacked planes that were subsequently crashed into the WTC and the Pentagon.
A NORAD spokesman, Lt. Col. Roberto Garza said they had given the commission all relevant documents. He said that 20 tapes of conversations involving NORAD personnel had been given to the commission, and the only material that was not handed over were another 28 tapes that were either blank or duplicates of the ones that had been handed over.
"We were surprised (by the subpoena)," he said. "We support the commission, we want to help them."
Commissioner Tim Roemer, a former Democratic congressman, told United Press International that the commission had also voted down a proposal to issue a subpoena to the CIA to obtain the so-called Presidential Daily Briefings, or PDBs, that the agency produces.
Roemer would not give further details of the vote.
The commission's mandate gives the chairman, former GOP New Jersey Gov. Thomas H. Kean, and his deputy, former Indiana Democratic congressman Lee H. Hamilton, the right to issue subpoenas after consulting other commissioners, but without a vote.
The PDBs, which are distributed early every morning to the president and a handful of top aides, summarize the most important threats to the nation. They are considered among the most secret and sensitive national security documents of all.
Roemer said that the commission had asked for PDBs and other presidential documents going back several years. He said they were essential if the commission was to complete its task of finding out what went wrong.
"We need to know what advice and warnings presidents Bush and Clinton might have received about al-Qaida," he said, "and what advice and warnings the intelligence community was issuing."
Media reports earlier this year suggested that an August 2001 PDB had warned about al-Qaida's plans to hijack U.S. jetliners.
Kristen Breitweiser, who lost her husband Ron in the attacks and who campaigned tirelessly alongside other victims' relatives for the commission to be established, said she was disappointed.
"These documents show the flow of information within the government, the chain from the bottom to the top," she said. "Something went wrong, the nation was unaware and undefended that day. If we want to find out what went wrong, we have to examine every link in the chain to find out where the break was."
Roemer said that the commission requested the documents on July 3. He said that there had been several weeks of negotiations with the White House about access to them. He said that so far, the commission was dissatisfied with the White House's posture.
"The offer that was on the table yesterday was too restrictive and threatened our independence," said Roemer, "It begs the question, 'what are they trying to hide?'"
"We don't want a repeat of the Warren Commission," he said, referring to the widely derided investigation into the assassination of President John F. Kennedy. "There must be no question marks, no asterisks, no stains on our report.
"The only way to ensure that is if the commission, not the White House or anyone else, decides who gets to see the documents we need."
Commission members and staff said the matter would remain a priority until it was resolved.
"The independence of the commission and its credibility in the eyes of the American people is essential. Personally, I am hopeful that a compromise can be reached which preserves that," said Ben-Veniste.
Neither Ben-Veniste nor Roemer would discuss details of the negotiations, or say what the White House offer was. But Ben-Veniste laid out what he said was a compromise.
"I have proposed that a subcommittee be established to review these ultra-sensitive materials. That's the minimum that would be acceptable to me personally."
"It's not a perfect arrangement," he went on, "it's a compromise, but we need to move forward given the urgency of our timetable."
The commission must report before a legislatively mandated deadline of May 27, 2004.
White House spokesman Scott McClellan told UPI that the administration was working with the commission, and had offered them "unprecedented cooperation."
"There are a number of ways we can provide the information the commission needs," he said, but he declined to give further details.
The New York Times reported Friday that the White House had offered to let the commission chairman and his deputy see the documents.
"Their offer was more restricted than that," said a commission source, asking not to be identified.
When asked about the reasons for the delay in handing over documents, McClellan said, "We don't want to do anything that might jeopardize national security or harm the war on terror."
Roemer responded by pointing out that the commission's requests related to the period before Sept. 11, when the war on terror had not yet begun, and that all commission members had security clearances high enough to see the documents.
Felzenberg said that there were some continuing problems with the rest of the Pentagon's response to document requests, but these were of a different order.
"They're practical questions," he told UPI, "connected with the physical transfer of this material. It's just a matter of getting things from here to there. We expect these to be solved very soon."
In a statement Friday evening, Undersecretary of Defense for Intelligence Stephen A. Cambone, who is coordinating the department of defense's response to the commission, said, "If there are any indications that a scheduled document delivery might not be met, the secretary (of defense, Donald Rumsfeld) expects that relevant department officials will inform me or him of that fact, and advise as to whatever resources may be needed to meet the schedule."