This has been said a lot of times in lots of places. Did the US government know about the attacks prior to the attacks, and were they involved with the attacks?
Depends on what you mean by prior knowledge, and what you mean by the U.S. Government.
The 9/11 Commission Report highlights many failings by the FBI and the CIA and suggests that they had credible information about the attacks even though that they did not realise what they had at the time. This could be taken as them having prior knowledge even if they didn't know about it.