Attorneys for 91-year-old John Demjanjuk have appealed to a U.S. court to hold a hearing on whether Demjanjuk should regain his U.S. citizenship, The Associated Press reported on Tuesday.

Demjanjuk, a retired auto worker, was found guilty in a Munich court last May of assisting in the murder of more than 27,000 Jews at the Sobibor concentration camp while serving as a Nazi guard.

His attorneys are now claiming that hundreds of pages of newly released documents cast doubt on the U.S. effort to revoke his American citizenship.

They charge that the government failed to disclose important evidence, including a 1985 secret FBI report uncovered by AP. The report indicates the FBI believed a Nazi ID card purportedly showing that Demjanjuk served as a death camp guard was a Soviet-made fake.

“Of course these documents are of great importance to the case,” the defense was quoted by AP as having said. “They cast doubt on the entire case. And they are created by the largest, most powerful law enforcement and investigative agency in the country.”

A spokesman for the U.S. attorney’s office told AP federal prosecutors won’t comment on the latest defense filing.

Demjanjuk, who denies serving as a guard at any camp, is free pending his appeal, AP noted. Though he currently cannot leave Germany because he has no passport, he could get a U.S. passport if the the appeal is discussed in court and the ruling is overturned.

Last month, the German government announced that it has launched a last-chance investigative effort on the dormant cases of hundreds of former guards and other personnel at Nazi death camps.

The investigations come in the wake of a new precedent set by Demjanjuk’s conviction.