Berlin ? The last remaining member of the Hamburg sleeper cell that planned and carried out the 9/11 terror attacks was sentenced to 15 years in prison in a German courtroom Monday.

Mounir el Motassadeq, a 32-year-old former engineering student from Morocco, received the harshest penalty possible under German law for the crime – accessory to murder for the 246 people who died on the four airliners that crashed on Sept. 11, 2001.

El Motassadeq was first convicted in 2003 of being an accessory to the deaths of all the victims of the Sept. 11 attacks, and he was sentenced to 15 years in prison. But a German federal court ruled on appeal that there wasn’t enough evidence to prove that el Motassadeq knew the scope of the attacks.

After years of legal wrangling – including a refusal by the Bush administration to allow testimony from alleged co-conspirators being held at the Guantanamo Bay prison in Cuba, which el Motassadeq said would vindicate him – German prosecutors reinstated the accessory to murder charge but narrowed the scope to include only those who died on the four jetliners.