(BBC) Some 70,000 people received weapons training and religious instruction in al-Qaeda camps, German police says.
The claim came at the retrial of Mounir al-Motassadek, a Moroccan man accused of involvement in the 9/11 attacks, which were partly planned in Germany.
A German police officer told the court recruits at the camps were taught they had a duty to kill US citizens.
Mr Motassadek says he received training in Afghanistan and knew some of the 9/11 hijackers – but not of their plot.
Although initially convicted of involvement in the attacks, he is now facing a retrial.
The case against him collapsed last year when a judge ruled that US authorities’ refusal to allow lawyers access to a key witness in their custody had prejudiced proceedings.
Giving evidence at the retrial, an officer from the German police body that co-ordinates the fight against terrorism said new al-Qaeda recruits were be stripped of their passports and taken to safe houses.
They also received a new name before being taken to al-Qaeda camps, most of which were located in Afghanistan.
An estimated 3,000 people died when al-Qaeda militants hijacked US passenger airliners and ploughed them into targets in New York and Washington on 11 September 2001.
Several key hijackers had lived in the German city of Hamburg and were referred to by investigators as the “Hamburg cell”.
Al-Qaeda’s training facilities in Afghanistan are believed to have largely been destroyed by a US-led invasion in late 2001.