At least six eyewitnesses saw the mysterious aircraft on the morning of September 11 last year. But the US authorities deny it ever existed.
So when George Bush laid a wreath yesterday at the crash site in a remote valley outside Shanksville, Pennsylvania, he was one of only a handful of people who know what really happened to the 40 innocents and four hijackers aboard the doomed United Airlines Boeing 757-200.
Those unimaginable final seconds as passengers showed courageous defiance apparently wrestling for control of the aircraft have become one of the defining images of the tragedy.
And "Let's roll" - ringleader Todd Beamer's no-nonsense call to arms - became a defining battle cry in America's war on terror.
But of the four aircraft taken on September 11, the exact fate of Flight 93 after its two-hour journey is proving difficult for US officials to explain.
What was the white jet doing there and why won't they admit to its presence? Why did other witnesses see smoke and flames trailing from Flight 93 as it fell from the sky, indicating a possible explosion aboard?
Or - and this is proving to be the most uncomfortable question of all - in the moments before the airliner piled into the black, spongey earth at 575mph did an American fighter pilot have to do the unthinkable and shoot down a US civil airliner?
Susan Mcelwain, 51, who lives two miles from the site, knows what she saw - the white plane rocketed directly over her head.
The Saddest Lie Of All