On 8 January 2007, he was sentenced to serve 15 years by the court of Hanseatisches Oberlandesgericht, Hamburg, because of 246 counts of accessory to murder in coincidence with membership in a terrorist organisation.
Not much is known of his activities at this time, but he did move into the Hamburg cell apartment owned by Mohamed Atta and lived with several other people who would later be accused by the U.S. and German authorities of leading the September 11, 2001, attacks.
[3] Mounir el Motassadeq was a friend of Mohamed Atta, Marwan al-Shehhi and Ziad Jarrah and "a member of their prayer group".
Though the German Justice Ministry pressed the United States to allow Ramzi bin al-Shibh to testify, the U.S. refused, and the verdict and sentence were set aside.
[8] On 15 November 2006, the German Federal Court of Justice ruled on the appeals: They considered the evidence as sufficient to prove that Motassadeq knew about and was involved in the preparation of the plan to hijack the planes and was hence guilty as an accessory in 246 counts of murder.
As of 2011, his lawyers were considering both appealing to the European Court of Human Rights and trying to get the case reopened – his two remaining legal options.
[10] Ahmad Wali Siddiqui, whose interrogation triggered a 2010 terror alert, was a friend of Motassadeq since 1997 who also patronized the mosque attended by many other alleged Hamburg-based 9/11 plotters.