Abdelghani Mzoudi, 31, barely reacted as the judge read out the verdict to the court, keeping his arms folded and looking down to the floor.

Federal prosecutors had been seeking the maximum 15 years in prison for more than 3,000 counts of accessory to murder, representing each of the lives lost on September 11, 2001, and membership of a terrorist organisation.

Similar evidence against Mounir el Motassadeq, a friend of Mr Mzoudi, secured a maximum sentence against him in February last year, in what was the first conviction in connection with the terror attacks.

The prosecutors alleged that Mr Mzoudi provided logistical support to the Hamburg cell under lead hijacker Mohamed Atta, helped with financial transactions and arranged housing for members to evade the attention of the authorities.

Mr Mzoudi spent time at a terrorist camp in Afghanistan in 2000.

Mr Mzoudi's defence lawyers denied the charges, arguing that while their client was a friend to many of the principle figures in the September 11 attacks, he knew nothing in advance of the plot to attack the United States.

The acquittal on all counts came after the court rejected a last-minute motion from a lawyer representing some of the relatives of American victims of the attacks.

Andrea Schultz said that his clients had access to "new information" from the US Department of Justice and urged the court to ask US authorities once again for statements made by Ramzi Binalshibh, a Yemeni believed to be the Hamburg cell's key contact with al-Qaeda.

Pressed by the court for details however, Mr Schulz could only say he was "not authorised" to tell the court what the evidence was. The judge rejected the motion.

US authorities have persistently refused access to the interrogation transcripts or Binalshibh himself, who has been in secret custody since his capture in Pakistan on the first anniversary of the September 11 attacks.

The Mzoudi verdict was originally scheduled for January 22, but prosecutors secured a last-minute delay to allow testimony by a man claiming to be a former Iranian intelligence agent, who implicated Mzoudi in the attacks.

The witness, identified by the alias Hamid Reza Zakeri, told German investigators he had information that Mzoudi directly took part in the September 11 plot, channelling information to others involved. Yet prosecuting lawyers conceded that the man's rambling testimony probably did not help their case.

The trial has illustrated the difficulty of using circumstantial evidence to prove involvement in the loose and secretive structure of al-Qaeda.