Mounir al-Motassadek, a 31-year-old Moroccan, admitted being close to the 11 September 2001 hijackers, but denied any knowledge of the plot to attack New York and Washington.
Motassadek's first conviction was quashed over access to evidence
In 2005, he received a seven-year jail term for membership of a terrorist group, after a year-long retrial in Hamburg.
But Germany's Constitutional Court ordered his release from prison in February 2006, pending the outcome of an appeal against his sentence.
At that appeal, in November 2006, he was convicted of being an accessory to more than 3,000 counts of murder.
At his first trial in 2003 he had also been sentenced to 15 years for being an accessory to the 9/11 plot, but that verdict was quashed on appeal.
It had been widely expected that Motassadek would be acquitted at the retrial, after his friend and fellow Moroccan - Abdelghani Mzoudi - was cleared on the same charges by the same Hamburg court in 2004.
For Motassadek's retrial, the US justice department released summaries from the interrogation of several al-Qaeda suspects, including Ramzi Binalshibh.
But defence lawyers criticised the lack of direct testimony from Mr Binalshibh and other witnesses. They argued that any information coming from the US government may have been got by torturing suspects, and was therefore unreliable.
The prosecution at the retrial had hoped US intelligence agencies would reveal more information about their sources.
Mr Binalshibh, a Yemeni citizen dramatically arrested in Pakistan and allegedly a key planner of the 9/11 attacks, told how Motassadek had taken part in vitriolic anti-US discussions at the home of Mohammed Atta, the alleged pilot of one of the hijacked planes.
But Mr Binalshibh also insisted that Motassadek was not aware of the 9/11 plot.
Motassadek allegedly handled Marwan al-Shehhi's bank account
Motassadek always said he was just one of thousands of young Muslims who came to Europe to study in the mid-1990s.
But he admitted having spent time at an al-Qaeda camp in Afghanistan and having signed Atta's will.
He was 19 when he came to Muenster in Germany, where he did a language course in 1993. He then joined Hamburg University to study electrical engineering.
During his time there, prosecutors in his original trial charged, he met Mohammed Atta.
While in Hamburg, Atta and several others founded a radical Islamic organisation, which fostered links with Osama Bin Laden's al-Qaeda network, prosecutors alleged.
They describe the group as having "a violent Islamic ideology aimed at attacking countries such as the United States".
Prosecutors said Motassadek "maintained intensive contacts over several years" with Atta and other 9/11 suspects Marwan al-Shehhi and Ziad Jarrah.
The members of the Hamburg cell arrived in Germany between 1992 and 1997 from various Arab countries, prosecutors alleged.
All, like Motassadek, came in order to follow further education courses.
Prosecutors said Motassadek acted as the group's "treasurer", handling funds for the living expenses of three of the hijackers - including Atta - who took flight lessons in the US for the 9/11 attacks.
Motassadek first came under suspicion in November 2001, when German authorities discovered that he had power of attorney over a bank account held by Shehhi, who allegedly flew the second plane into the World Trade Center.
He was questioned and released, but remained under police surveillance.
He went on trial in Hamburg in October 2002.
Between 1996 and 1998 Motassadek worked as a cleaner at Hamburg airport. He passed a routine security check allowing him to work in secure areas and aircraft.
In the summer of 1998 he got a job at a Volkswagen factory.
He was also involved in the running of the al-Quds mosque in Hamburg, which prosecutors say was the focal point of the group's activities.
Motassadek is understood to be married to a woman from Belarus, with whom he has two children.
Following his arrest in November of 2002, university officials described Motassadek as a "normal student" eager to take his exams despite the allegations against him.