Mohamed Atta was an Egyptian student who studied in Germany in the late 1990s and soon became friends with Ziad Jarrah, Marwan al-Shehhi and Ramzi Binalshibh who together formed what is now known as The Hamburg Cell. In the late 1990s, the four friends became known associates of al-Qaeda, and the ringleaders of the 19 hijackers in the September 11 attacks. Atta was the hijacker in control of American Airlines Flight 11 which was the first plane to strike the World Trade Center.
http://www.abendblatt.de/hamburg/article175063/Neue-Mieter-in-der-Marienstrasse-54.html