Phoenix Memo

The Phoenix Memo is a letter sent to FBI headquarters on July 10, 2001 by FBI Special Agent Kenneth Williams, recommending the assembling of a worldwide listing of civil aviation schools. Williams, then stationed in Phoenix, Arizona, was at the time investigating students at some of these schools for possible terrorist links.
Content.
According to Williams, the purpose of the memo was to
advise the Bureau and New York of the possibility of a coordinated effort by Osama Bin Laden to send students to the United States to attend civil aviation universities and colleges. Phoenix has observed an inordinate number of individuals of investigative interest who are attending or who have attended civil aviation universities and colleges in the State of Arizona.
The recommendations outlined by Williams were ignored or put aside because of other concerns. The memo was seen by at least one dozen officials of the FBI, including John P. O'Neill, but it was never passed to acting director Thomas J. Pickard, his successor Robert Mueller, or the Central Intelligence Agency. In addition, the existence of the memo was not made known to President George W. Bush and his senior national security staff until May 2002. 
Mueller told Congress in the hearing in May 2002, that failure to act on the memo was due to deficits in the Bureau’s analytical capabilities.
Coleen Rowley (2002).
The memo became the subject of another communiqué in June 2002, when FBI Agent Coleen Rowley took advantage of the federal Whistleblower Protection Act provisions to inform FBI Director Robert Mueller that his public statements about lack of “advance knowledge” by the Bureau have no basis in facts. In her memo, Rowley wrote about the alleged suppression of the investigation concerning Zacarias Moussaoui.
Analysis.
Bill Gertz of "The Washington Times" suggests that the lack of attention to the memo is one of several intelligence failures, leading to the September 11, 2001 Attacks.