What you are referring to is a conspiracy of silence. The more people involved in the secret, the less likely the secret will be kept. How many people have to be involved to execute a totally fake mass casualty event? Dozens? How do you keep them silent? Blackmail? Would anyone believe them even if they wanted to talk? These are interesting questions. Logistically speaking, faking the entire event seems like a less than ideal approach - certainly not impossible but poses many risks and opportunities for it to be exposed