JFK was shot by an American
Reagan was shot by an American
An American blew up a building in Oklahoma
I'm not 100% sure on this one, but I think it was an American sending anthrax by mail to people
Remember the American guy who was fighting with the Taliban? That was funny
Long ago, but still, Lincoln was shot by an American

There's probably more but I can't be bothered to look it up....

What message does this send? That Americans will eventually self-destruct by means of terrorist acts directed against themselves? Oh, the irony....