After the September 11 attacks in 2001 , the FBI released a letter reportedly handwritten by the hijackers and found in three separate locations on the day of the attacks — at Dulles International Airport , at the Pennsylvania crash site , and in hijacker Mohamed Atta 's suitcase . It included a checklist of final reminders for the 9/11 hijackers . An excerpt reads : " When the confrontation begins , strike like champions who do not want to go back to this world . Shout , ' Allāhu akbar , ' because this strikes fear in the hearts of the non-believers . " Also , in the cockpit voice recorders found at the crash site of Flight 93 , the hijackers are heard reciting the Takbīr repeatedly as the plane plummets toward the ground and the passengers attempt to retake control of the plane
Pennsylvania crash site
United Airlines Flight 93
Takbir