An American Airlines 737 was hijacked from Boston and crashed minutes later into tower two. There could be 130-180 people aboard.
Fifty thousand people work in the twin towers. Assuming both planes hit midway with twenty-five thousand people above the impact points, the fires (and more deadly) the asphyxiating smoke could get a number of them. A wild-assed guess could be anywhere from five to fifteen thousand.
An attack on the Pentagon has occured and I have no idea what that did yet.
In short, someone has probably killed in the neighborhood of ten thousand Americans to make a point. Are you impressed?
NaW.
(They've certainly got my attention...)