The Hamburg cell was, according to U.S. and German intelligence agencies, a group of radical Islamists based in Hamburg, Germany, that included students from different Arab countries who eventually came to be key operatives in the 9/11 attacks. Important members included Mohamed Atta, who led the four hijacking teams in 2001 and piloted American Airlines Flight 11; Ramzi bin al-Shibh, who conspired with the other three members but was unable to enter the United States; Marwan al-Shehhi, who piloted United Airlines Flight 175; and Ziad Jarrah, who piloted United Airlines Flight 93 and failed to hit a target in Washington, D.C.. Other members included Said Bahaji, Zakariya Essabar, Mounir el-Motassadeq, and Abdelghani Mzoudi.

Ramzi Mohammed Abdullah bin al-Shibh is a Yemeni citizen currently being held by the U.S. as an enemy combatant detainee at Guantanamo Bay in Cuba. He is accused of being a "key facilitator for the September 11 attacks" in 2001 in the United States.