Given the following lessons from history, how can we not launch a new investigation into what really happened on 9/11?
It is widely accepted that the Nazis, in Operation Himmler, faked attacks on their own people and resources which they blamed on the Poles, to justify the invasion of Poland.
It has now been persuasively argued — as shown, for example, in this History Channel video — that Nazis set fire to their own government building and blamed that fire on others (if you have trouble playing the clip, it is because the website hosting the clip requires you to download the clip before playing it). The fire was the event which justified Hitler's seizure of power and suspension of liberties.
In the early 1950s, agents of an Israeli terrorist cell operating in Egypt planted bombs in several buildings, including U.S. diplomatic facilities, then left behind "evidence" implicating the Arabs as the culprits (one of the bombs detonated prematurely, allowing the Egyptians to identify the bombers). Israel's Defense Minister was brought down by the scandal, along with the entire Israeli government. See also this confirmation.
The Russian KGB apparently conducted a wave of bombings in Russia in order to justify war against Chechnya and put Vladimir Putin into power (see also this short essay and this report).
The Turkish government has been caught bombing its own and blaming it on a rebel group in order to justify a crackdown on that group.
The well-respected former Indonesian president said that the government had a role in the Bali bombings.
As documented by the New York Times, Iranians working for the C.I.A. in the 1950's posed as Communists and staged bombings in Iran in order to turn the country against its democratically-elected president (see also this essay).