I have just finished reading a book by Donn de Grand Pre which says that the Global Hawk was responsible for the 9-11 hits on the World Trade Center buildings. This from a search on bing.com:
According to the Air Force, the capabilities of the aircraft allow more precise targeting of weapons and better protection of forces through superior surveillance capabilities.
However, this does not explain why WTC 7 collpased. I suspect it may have been Mossad truck bombs which caused this to occur.