9/11 has provided many people with an interesting intellectual puzzle, the solution to which, in outline at least, is fairly obvious, but which will never be publicly acknowledged short of a coup d'état in Washington, DC.
The reason the truth cannot otherwise be acknowledged is obvious: many people were involved in 9/11, which means that public recognition of the truth would necessitate mass trials and executions of members of both Democratic and Republican Party leadership, plus many prominent persons within the security apparatus. Almost certainly, corporate entities, their directors, and members of their technical staffs would also be implicated, together with security services and corporate entities of several other countries.