How was the Patriot Act written and then passed in only 6 weeks?

I'm curious to hear what others may think when it comes to the Patriot Act. That is a lot of legislation. When some bills take years to write, and the pass in the Senate or Congress, how was this bill written so quickly, and then passed through the Senate and Congress even quicker. My suspicion was that it was written long before 9/11, which doesn't necessarily mean that there's a conspiracy here...I'm sure a lot of bills are written and then shelved for periods of time. Does anyone know the true history of this act?