Marwan al-Shehhi is thought to have piloted United Air Lines Flight 175 into the World Trade Center's south tower.

CNN: Uncle calls hijack suspect 'innocent passenger'

Atta and a second suspect, Marwan Al-Shehhi, also studied in the city's Harburg Technical University, in Hamburg, German prosecutor's say.

CNN: suspect

Mohamad Atta, the suspected leader of the hijack gangs, Ziad Samir Jarrah and Marwan al-Shehhi, both of whom had flying lessons in Florida, had all spent time in Hamburg.

CNN: Germany tightens security net