A German court has heard evidence that a Moroccan man, Abdelghani Mzoudi, helped the 11 September hijackers by sending coded information to middlemen.
Mr Mzoudi admits knowing the hijackers but not their plans
The information comes from someone claiming to be a former Iranian agent, whose identity is being kept secret.
It arrived just as the trial was due to end - the verdict has now been delayed by one week to next Thursday.
Mr Mzoudi faces charges of being an accessory to murder and part of a terrorist group.
Prosecutors say he helped the Hamburg cell responsible for the 11 September attacks, which killed more than 3,000 people, and want him to be given the maximum sentence of 15 years in jail.
The new witness claims Mr Mzoudi was responsible for "drawing up and sending information to middlemen, because he was knowledgeable with codes," according to a transcript of his interview given to the court.
He also claims to have tried to warn the United States before September 2001 that "something would happen".
Trial Judge Klaus Ruehle read extracts from the interview in court on Thursday, according to the Associated Press.
He also asked a German federal investigator, who interviewed the man, to assess their credibility.
But investigator Andy Neumann, giving evidence in court, said he was unable to do so immediately.
Ulrich von Jeinsen, a lawyer for American relatives of victims of the attacks who are co-plaintiffs in the case, said the witness's evidence was "sensational" and the trial was "a whole new ball game".
The prosecution case against Mr Mzoudi appeared to have all but crumbled last month when the court released Mr Mzoudi from custody on the basis of new evidence.
That evidence came in the form of a statement from an unidentified informant saying that there were only four people in the Hamburg cell responsible for the attacks - three pilots and Ramzi Binalshibh who is already in US detention.
The source was not named but the court said it believed it was Mr Binalshibh.
Mr Mzoudi is the second person anywhere in the world to be tried over the attacks.
Moroccan Mounir al-Motassadek was jailed in Germany in February as an accessory to more than 3,000 murders in New York and Washington as a result of 11 September.
The court which convicted Motassadek heard that he was a member of the Hamburg cell which planned the terror attacks.
He was jailed for 15 years, but is now appealing against his conviction.