See also Joint Inquiry
The FBI, or Federal Bureau of Investigation, in theory would have been one of the ideal agencies to investigate 9/11, gather the clues, follow the trail of evidence, and find out just what happened on 9/11 and who was likely behind it. That’s what the I stands for in FBI: Investigation. They did not. The FBI’s actions before 9/11 suggest very strongly that they knew about the hijacker plot and did not want to stop it. In fact investigations in Chicago (by FBI agent Robert Wright), Minneapolis (see Colleen Rowley), Phoenix (the Moussaoui case and the Phoenix memo) and Washington, D.C. (the Able Danger program) were all shut down or at the very least obstructed or ignored by FBI headquarters in Washington. Any one of these independent investigations might have enabled the FBI to prevent the attacks of 9/11.
After 9/11 the FBI hired Sibel Edmonds as a translator. This is a long story but to make it short, Edmonds noticed that a co-worker was protecting certain foreign governments against really being spied on by the FBI. This coworker specifically asked Edmonds to let her handle the translation of all documents relating to those governments. When Edmonds smelled a rat, being a principled and honest person who took seriously her oath of citizenship and her responsibility to the country, and spoke up, she was blackballed. You can read her whole story on her website www.justacitizen.com
It’s very interesting to note that on the FBI website, on their page for most wanted terrorists, there is Osama bin Laden, whom the government blames for being the overall leader of al Qaeda and responsible for the attacks of 9/11. Yet bin Laden is not wanted by the FBI for 9/11. An FBI agent has said words to the effect of “We do not have evidence connecting bin Laden to 9/11”.
Like the other intelligence agencies, the FBI could have and should have cooperated with every investigation that’s been conducted by the government on 9/11. They should have also been responsive to FOIA (Freedom of Information Act) requests by citizens. But the FBI obstructed the Joint Inquiry, Congress’ 2002 investigation into the intelligence agencies’ roles in 9/11, by denying or delaying access to documents and people. (So did the CIA, the Department of Justice and other agencies in the intelligence community.)  This is all documented by the Joint Inquiry in the appendix titled “Access Limitations Encountered by the Joint Inquiry” starting on page 814 of the 838 page Final Report.  You can download the final report from this page at the previous link or from the government’s  website.  Click on where it says “pdf” to the right of “download files”.
As 2012 draws to a close the FBI has created a large track record of creating terrorist plots in order to stop them, using entrapment.  On the surface it may appear, and the FBI claims, to have stopped another terrorist attack.  Below the surface one can see the FBI’s real intention of undermining any opposition to the global war on terror.  In the process they violate our constitutional rights, infiltrate peace groups who have done nothing illegal, and turn America into a fascist state, a corporatocracy.  Details about this pattern of illegal, counterproductive, unconstitutional and blatantly dishonest law enforcement are brilliantly reported in this article by Ben Swann in his series “Reality Check”.  The title of this article is “Reality Check: Is the FBI making us safer breaking up terror plots?”
Related to this, former whistleblower and UL employee Kevin Ryan has written a very detailed article titled “Why Louis Freeh Should Be Investigated For 9/11“.  It describes a long pattern of the FBI’s failures under former Director Louis Freeh.
In short the FBI could and should be part of the solution (including prevention) on 9/11 but the top leadership of the FBI, contrary to these brave and honest people, has instead been part of the problem.