BERLIN—German prosecutors are investigating Audi AG Chief Executive Rupert Stadler and another unidentified member of the car maker’s executive board on suspicion of fraud as they widen their investigation into Volkswagen AG’s emissions-cheating scandal.
The investigation is a blow to Mr. Stadler, a longtime executive at Audi and its parent Volkswagen, who has repeatedly denied any knowledge that diesel engines made by the companies were rigged to cheat on emissions tests before the practice was disclosed by U.S. authorities...