>> reporter: 17 years ago, in the hours after the attacks, air travel coast to coast was suspended. the skies fell quiet for the better part of three days, and every year since, the silence has returned. with bowed heads and broken hearts, the whole nation has watched survivors and their wrestle with their grief, seeing the landscape of the tragedy transformed, even watched the presidency change hands, but the message has stayed the same. >> this field is now a monument to american defiance. this memorial is now a message to the world. america will never, ever submit to tyranny. >> reporter: those are not just words. terrorists who planned that terrible day were hunted fiercely in afghanistan, indeed, around the world, and their