I found this interesting. I'm not saying Sept 11 was done by the government, but this does make you think.
http://www.gwu.edu/~nsarchiv/news/20010430/In his new exposé of the National Security Agency entitled Body of Secrets, author James Bamford highlights a set of proposals on Cuba by the Joint Chiefs of Staff codenamed OPERATION NORTHWOODS. This document, titled Justification for U.S. Military Intervention in Cuba was provided by the JCS to Secretary of Defense Robert McNamara on March 13, 1962, as the key component of Northwoods. Written in response to a request from the Chief of the Cuba Project, Col. Edward Lansdale, the Top Secret memorandum describes U.S. plans to covertly engineer various pretexts that would justify a U.S. invasion of Cuba. These proposals - part of a secret anti-Castro program known as Operation Mongoose - included staging the assassinations of Cubans living in the United States, developing a fake Communist Cuban terror campaign in the Miami area, in other Florida cities and even in Washington, including sink[ing] a boatload of Cuban refugees (real or simulated), faking a Cuban airforce attack on a civilian jetliner, and concocting a Remember the Maine incident by blowing up a U.S. ship in Cuban waters and then blaming the incident on Cuban sabotage. Bamford himself writes that Operation Northwoods may be the most corrupt plan ever created by the U.S. government.
And now a link to the actual government docs.
http://www.gwu.edu/~nsarchiv/news/20010430/northwoods.pdf