Federal prosecutors said in a statement today they believe Trinh Xuan Thanh was kidnapped July 23 in the German capital.
Vietnam had requested the extradition of Thanh, 51, on embezzlement charges, which carry the death penalty.
He'd sought asylum in Germany and German authorities kicked Vietnam's intelligence attache out of the country after the alleged kidnapping and summoned the ambassador.
Vietnam last week said Thanh had turned himself in, calling it "regrettable" that Germany accused Hanoi of kidnapping.