Ulrich Busch said the Federal Bureau of Investigation had said in a 1985 report that an SS identity card alleged to be Demjanjuk's was "quite likely fabricated" by the Soviet Union.
"We have always maintained that the ID card was a forgery. Now we have a report from the FBI," Busch said.
He said he was in possession of the report and had applied to the court in Munich, southern Germany, to suspend the 91-year-old's trial to travel to the United States to look in the archives for more information.
However, he was pessimistic as to his chances of getting the court to agree to his request.
"The court has long since made up its mind," he said.
The identity card is vital to the prosecution's case as there are no living witnesses able to prove the charge that Ukrainian-born Demjanjuk was a guard at the Sobibor death camp in 1943.
Demjanjuk is charged with helping to murder 27,900 Jews and others during his alleged six-month stint at the camp where the Nazi death machine killed around a quarter of a million people.
Prosecutors have called for him to spend six years behind bars for his alleged actions in what is quite likely to be the last major trial dealing with the crimes committed in World War II.
A verdict could come as soon as May 12, although this has repeatedly been delayed due mainly to the defendant's health.