Depends on your definition of U.S. soil. I consider US soil to be, as most people around the world, any locations within the geographical US, military installations, and diplomatic facilities. That said, there were were at least 10 incidents that meet the above criteria. There were several more involving either direct attacks on U.S. citizens or at random. I counted at least 2 within the geographical confines of the US - one was a Palestinian shooting people at the Empire State Building and one was letter bombs sent to a NY newspaper from Alexandria.