Mounir al-Motassadek, a Moroccan, was the only person anywhere to have been convicted in connection with the 11 September attacks. But in 2004, Germany's Supreme Court threw the verdict out and ordered a retrial a year after his conviction.
The top court ruled he had been denied a fair trial because the US refused his defence access to a suspect Washington is holding.
A Hamburg court had earlier found him guilty of involvement in the 11 September attacks on the World Trade Center in New York and the Pentagon in Washington.
He was convicted of being an accessory to more than 3,000 counts of murder and of being a member of a terrorist organisation.
He was sentenced to 15 years in prison - the maximum penalty that could be imposed for the charges.
Throughout the trial Motassadek insisted he was innocent - that he knew nothing about the attacks, and knew the hijackers only socially.
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