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They recovered four, three of them AT the crash sites! How. The black boxes didn't even survive. 
Okay 9/11 truthers, since it seems that there's a lot of you here, why would the Bush administration do it? To justify an invasion? We could have invaded anywhere we wanted any time we wanted. Thinking that we needed a lot of people to die on domestic soil in order to justify an invasion in the public eye is lib shit. So what's left? Was the government whacking someone in the towers?