Mounir El Motassadeq was sentenced to 15 years in February 2003 after being found guilty of 3,066 counts of accessory to murder and of assisting members of the al-Qaida cell that included three of the Sept. 11 suicide hijackers: Mohamed Atta, Marwan al-Shehhi and Ziad Jarrah.
El Motassadeq had power of attorney for al-Shehhi and managed his bank account, which prosecutors argued provided funding for the terror cell and paid for the hijackers' flying lessons in the United States.
The 29-year-old Moroccan also signed the will of Atta, the suspected ringleader of the attacks that killed nearly 3,000 in New York, Washington and Pennsylvania. He acknowledged knowing several of the other attackers and organizers, but denied being familiar with anything of their plans.
During his three and a half-month trial, El Motassadeq, an electrical engineering student, maintained his innocence.
His lawyers have argued that his trial was flawed because the United States would not allow testimony by Ramzi Binalshibh -- thought to be the Hamburg cell's key contact to al-Qaida -- whom El Motassadeq said could support his claims of innocence.
Binalshibh was captured in Pakistan on the first anniversary of the Sept. 11, 2001, attacks and is being held by the United States in an undisclosed location.
In addition, the German government refused to release transcripts of Binalshibh's interrogations, saying the United States provided them only for intelligence purposes.
Presiding judge Klaus Tolksdorf said Thursday the state could not abandon principles of justice, however grave the crime.
"The fight against terrorism cannot be a wild, uncontrolled war," he said, according to Reuters.
Following the decision, El Motassadeq's lawyers said they would ask the Hamburg court to free the engineering student.
The ruling is the second setback for prosecutors after the same Hamburg court last month acquitted El Motassadeq's friend Abdelghani Mzoudi of identical charges for lack of evidence.
Mzoudi's case was helped by a statement presented by German investigators in which an unnamed source, believed to be Binalshibh, said the only people in Hamburg who knew of the Sept. 11 plot were hijackers Atta, al-Shehhi and Jarrah, and Binalshibh.
Though that evidence was not considered in El Motassadeq's appeal, defense lawyers said it would "definitely" play a role in the retrial, according to the Associated Press.
-- Compiled from wire reports and other media sources