Germany's highest court of appeal rejected a legal challenge by a Moroccan friend of the September 11, 2001 hijackers against a 15-year jail sentence for being an accessory to mass murder.

The Federal Court of Justice said on Friday it had thrown out the appeal by Mounir El Motassadeq on the grounds that it was unjustified. Motassadeq's lawyers had complained his sentencing contained legal errors, but the court disagreed.

In January, Germany's Federal Constitutional Court rejected a separate appeal made by Motassadeq about the 15-year sentence handed down by a court in Hamburg earlier that month.

Motassadeq was a member of a group of radical Arab students in Hamburg, led by Mohammed Atta, which helped organize the 2001 attacks on the World Trade Center and the Pentagon.

The complex and drawn-out case strained Berlin's relations with Washington as German courts tested how far the United States would go in giving sensitive evidence.