The US will not allow German judges to question al-Qaeda prisoners in the retrial of a man accused of complicity in the 11 September attacks.
When convicted last year, Mounir el-Motassadeq was the only person to have been found guilty in connection with the attacks.
The 30-year-old Moroccan was sentenced to 15 years in prison, but in March his conviction was quashed because a potentially crucial witness who was in US custody was kept from testifying.
It has been reported that Ramzi Binalshibh told US interrogators that al-Qaeda's Hamburg cell - which played a central role in the attacks - consisted only of himself and three suicide hijackers and not Mr Motassadeq.
The retrial began in Hamburg this morning but the presiding judge said US authorities were refusing to allow what they termed interactive access to al-Qaeda suspects.
Mr Motassadeq is charged with complicity in the murder of about 2,800 people in the attacks and membership of a terrorist organisation.
