America's military top brass may have deliberately misled the 9/11 commission, painting a picture of a swift response to the unfolding terrorist attacks when, in reality, fighter jets spent crucial minutes pursuing a non-existent plane, it emerged yesterday.

Tape transcripts published for the first time in this month's Vanity Fair magazine show in vivid new detail how air defence commanders sent two armed planes - out of only four at their disposal to defend a quarter of the country - on a wild-goose chase. Meanwhile, contrary to what the commission was told, the military did not even hear about United flight 93 until after it had crashed.

Members and staff of the 9/11 commission believe the discrepancies may have been part of a deliberate effort to mislead, and they discussed asking the justice department to consider criminal charges against senior members of the military, sources told the Washington Post.

Thomas Kean, the former New Jersey governor who led the commission, described what his panel had been told by the North American Aerospace Defence Command as "just so far from the truth". To this day, he said, commission members still "don't know why Norad told us what they told us". John Farmer, a senior counsel to the commission, told Vanity Fair that the military's story was "a whole different order of magnitude than spin. It simply wasn't true".

Major General Larry Arnold and Colonel Alan Scott told Mr Kean's panel that Norad started tracking United 93 at 9.16am. In fact, the plane was not hijacked until 12 minutes after that and, as the tapes grimly illustrate, it was 10.15 before technical specialist Sergeant Shelley Watson, a Norad employee, first heard the news from civilian air traffic control in Washington.

"He's down," an air traffic controller tells her on the tape, though she at first misunderstands. "When did he land? Because we have confirmation ..."

"He did ... he did ... he did not land," the controller replies.

"Oh, he's down down?" Sgt Watson replies, as the truth dawns.

The tapes seriously damage the credibility of the claim, made repeatedly by the vice-president, Dick Cheney, that the White House had weighed the question of giving authorisation for flight 93 to be shot down if necessary. In fact, the vice-president knew of the hijack barely a minute before the plane crashed. The authority to give the order was provided by the president at 10.18 - after the attack had ended.

They also provide further evidence against the conspiracy theory that the military shot the plane down: the tapes show Norad could never have been in the position to do so. In fact, two fighter jets wasted part of the morning pursuing a ghost: Norad chiefs believed that American flight 11 had fallen beneath their radar and was heading to the US capital, even though it had already crashed into the World Trade Centre. A crucial part of the problem, one civilian aviation manager said, was that American Airlines refused to confirm for several hours that its flight 11 had hit the building.

"American Airlines is still airborne - 11, the first guy," Major Kevin Nasypany says at one point. "He's heading towards Washington. OK, I think we need to scramble Langley right now. And I'm - I'm gonna ... chase this guy down if I can find him."

Colonel Robert Marr, the commanding officer in charge of Norad's north-eastern division, said no deception had taken place. "I can't think why we'd want to spin that," he told Vanity Fair.

In the end, the 9/11 commission did not refer the Norad officials to the justice department but passed the matter to the inspector-generals of the Pentagon and transportation departments, who act as government watchdogs. Both departments said reports were forthcoming.

by Oliver Burkeman in New York