Not to mention that the Big 7 of the DCU were in full swing at that time. The minute they hear the words 'terrorist', 'hijack' and 'plane' they would have teleported to that exact spot and stopped anything bad from happening. There is also the fact that technology is likely to be more advanced in the DCU and MU than in our world, what with the constant crisis's and somesuch, there would have been technology advanced enough to protect the lives of Firemen, Ambulances, Medical personal, etc . Moreover, the JLA have a mind reader, a warrior with a lasso that can compel people to tell the truth, the world's greatest detective, that would have hunted down all parties responsible in a matter of days or weeks and not to mention the said warrior is also a diplomat that can defuse political tension.