I doubt many know THE truth, or whole story about 9/11. Those who planned it had no reason to tell everything to those assisting. They could have easily compartmentalized the plan among the helpers.

Intelligence agents and insiders have no incentive to speak up about what they know:a) they would lose their secrecy and their payb) there is no reason to believe anyone would take them seriously (do you believe Barbara Walters or anyone on mainstream TV would interview Todd Beamer, said to have died on Flight 93?)c) those who speak up or are thought to be about to speak up often wind up dead or humiliated (consider the mysterious death of Barry Jennings, for one)