The Bush administration had foreknowledge of the attacks and actively suppressed attempts to stop them. George W. Bush was close personal friends with one of the key architects of 9/11 and afterwards great measures were taken to cover up this person's involvement (the so-called "28 pages").
A lot of other people would cite the Gulf of Tonkin as an example too, but that's something I know less about so I'll leave it to someone else.
The Gulf of Tonkin incident was explained to the American public at the time as pair of attacks on two separate nights against the US Navy by the North Vietnamese. This was used as the casus belli for the Vietnam War; in response to the two attacks congress passed the Gulf of Tonkin Resolution which authorized LBJ to start a war (without formally declaring a war...)
The first attack probably did happen, however in 2003 Robert McNamara (who had been the US Secretary of Defense at the time of the incident) admitted the second attack never happened. Straight from the horse's mouth: https://www.youtube.com/watch?v=HODxnUrFX6k
No, it was used as the excuse for yet another escalation of the Vietnam War, which the US had been overtly been involved in since immediately after the French gave up in 1954, and had significantly escalated it's involvement in (again, overtly) throughout the Kennedy Administration.