Actually, the terrorist plan that manifested itself into 9-11 was known by the DOJ going back to the World Trade Center bombing trials at USDC SDNY...I have the transcripts that detail that the FBI confiscated detailed plans from Ramsey Usef.
Moreover, David Schippers tried in vain to warn the Bush Adminstration about a terrorist plan to fly commercial aircraft into buildings in downtown Manhattan:
http://www.wnd.com/news/article.asp?ARTICLE_ID=25008