From Jesse Ventura's book, 63 Documents The Government Doesn't Want You to Read (full audio book for free here):
Two months before the events of September 11th, 2001 [July 10th 2001], an FBI agent in Phoenix named Kenneth Williams sent a memo to the bureau brass in D.C. and New York. The agent was warning about an unusually high number of Muslims being trained at American flight schools, perhaps part of "a coordinated effort" by Osama bin Laden. His memo was ignored at the higher levels.
The stories of Sibel Edmonds, Robert Wright, Coleen Rowley and Harry Samit, the "Phoenix Memo," David Schippers, the 199i orders restricting investigations, the Bush administration's order to back off the Bin Ladin family, the reaction to the "Bojinka" plot, and John O'Neil do not, when considered in sum, indicate mere incompetence, but high-level corruption and protection of criminal networks, including the network of the alleged 9/11 conspirators. (Nearly all of these examples were omitted from or relegated to fleeting footnotes in The 9/11 Commission Report.)
And this is only the tip of the ice berg. To dig deeper, visit our 9/11 whistleblowers page.