As the second airliner slammed into the south tower of the World Trade Centre, Vice-President Dick Cheney was staring at a television in the White House. It was 9.03am. His Secret Service men grabbed him and hurried him down to the President's emergency operations centre, an underground bunker hardened to withstand a nuclear attack
On the way to the tubular structure, Mr Cheney was told that another plane, or a helicopter loaded with explosives, was headed for the White House. He promptly called the President in Florida, who had just boarded Air Force One, and urged him not to come back to Washington immediately.
In the bunker, the Vice-President was joined by Condoleezza Rice, the national security adviser, and the Transportation Secretary, Norman Mineta, among others. They were told six commercial aircraft were unaccounted for, all of which were potential missiles. One had supposedly crashed in Kentucky (not true), and another in Pennsylvania (accurate; its passengers or crew, apparently struggling with the hijackers, may have saved the White House).
The airliner that had taken off at Dulles AA Flight 77 did a 360-degree turn, away from the White House and, at 9.45am, slammed into the Pentagon.
At about that time, accounts began coming into the White House bunker that four international flights were headed toward Washington over the Atlantic and another from Korea. It could not be determined whether they were hostile; part of the terrorist scheme. US fighters and an Awacs control aircraft scrambled.
A threatening message received by the Secret Service was relayed to the agents with the President that "Air Force One is next." American code words were used showing a knowledge of procedures that made the threat credible.
Karl Rove, the president's senior adviser, told William Safire of the New York Times:"When the President said 'I don't want some tinhorn terrorists keeping me out of Washington,' the Secret Service informed him the threat contained language showing the terrorists had knowledge of his procedures and whereabouts. It was decided to get airborne with a fighter escort."
After the President landed at an air force base in Louisiana and made a tape for broadcast, he was, in Mr Rove's words, "pretty antsy" about not being at the centre of command.
George Bush made clear to the Vice-President his intense desire to return to Washington. The Secret Service objected strongly. Mr Cheney, a former Defence Secretary, suggested Air Force One go to Offutt base in Nebraska, headquarters of the Strategic Air Command, where the president could convene the National Security Council.
A White House official, quoted by Mr Safire, said "It would have been irresponsible of him to come back, pounding his chest when hostile aircraft may have been headed our way. Any suggestion that he should have done so is ludicrous."
The most puzzling aspect of the events concerns the credibility of the "Air Force One is next" message. It was clearly viewed as a threat, not a friendly warning but if so, why would the terrorists send it? It is also unclear how they got the codeword information and transponder know-how.
The worry now must be that knowledge of codewords, presidential whereabouts and secret procedures indicates the terrorists may have a mole in the White House - or the Secret Service, FBI, FAA or CIA.
If so, America's war on terror may well have to start in its own front room.