Motassadeq first came to Germany in 1993 and moved to Hamburg in 1995, where he studied electrical engineering in college. Little is known of his activities at this time, but he did move into the Hamburg cell apartment owned by Mohammed Atta and lived in by many other people who would later go on to lead the September 11, 2001 attacks.
On May 22, 2000, Motassadeq flew to Istanbul, and from there to Afghanistan. He soon returned. When the four 9/11 leaders went from Germany to Afghanistan to train, Motassadeq remained in Germany. German police were able to wiretap Motassadeq, but apparently did not discover any incriminating information. He maintains his innocence. "There never was a terrorist organization in Hamburg," he said.
At Motassadeq's trial, Aysel Sengün, the girlfriend of one of the alleged hijackers, Ziad Jarrah, was called to testify her relationship with Jarrah and his role in the plot. He is said to be friend of three of the hijacker pilots and "a member of their prayer group". Though he continuously denies any advance knowledge of the attacks on New York and Washington.
In February 2003, he was convicted in Germany of over 3,000 counts of accessory to murder. He was convicted in direct relation to the September 11 attacks, but the conviction was rejected on appeal. Though the German Justice Ministry pressed the United States to allow Ramzi Binalshibh to testify, the U.S. refused, and the verdict and sentence were set aside.
On February 7, 2006, Germany's Federal Constitutional Court ordered an early release of Motassadeq. The highest court of Germany ruled there was an absence of proof that Motassadeq was informed about the September 11 terrorist plot.
On November 15, 2006, the German Federal Supreme Court 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 is hence guilty of accessory in 246 counts of murder. This is the number of victims that died in the planes but does not include the victims on ground. The Oberlandesgericht (state supreme court) in Hamburg then took up the trial again in order to decide on the sentencing. Two days later, the Federal Supreme Court also revoked the release order and Motassadeq was arrested again. On January 8, 2007, he was sentenced by the Oberlandesgericht Hamburg to 15 years in prison, the maximum sentence possible under German law. The Federal Constitutional Court of Germany did not accept to revise his case. On May 2 the Federal Court of Justice of Germany rejected a plea for revision. His lawyers are currently thinking about both calling upon the European Court of Human Rights and trying to get the case reopened - his two ultimate legal choices left.