Prosecutors said on Monday that they were also investigating another member of Audi’s top management. They did not identify the person, in line with German rules designed to shield people who are not considered public figures. However, two people with direct knowledge of the investigation confirmed a report in the Bild newspaper that the second suspect was Bernd Martens, head of purchasing for Audi.
Audi said it was cooperating fully with investigators but declined to comment further. Volkswagen also declined to comment.
The investigation of Mr. Stadler, who has not yet been charged with any crimes, is likely to intensify criticism that Volkswagen has taken too long to replace executives who were part of the system that allowed the cheating to take place.
Mr. Stadler, 55, joined Audi in 1990 and was later chief of staff for Ferdinand Piëch, a legendary but feared former chief executive of Volkswagen. Mr. Piëch was credited with building the company into one of the world’s largest carmakers, but he was also blamed for creating a win-at-all-costs culture that nurtured the deception.
Mr. Piëch, who no longer exerts any direct influence at Volkswagen, is a grandson of Ferdinand Porsche, the designer of the Volkswagen Beetle. Mr. Stadler’s close connection with the Porsche family, which owns a majority of Volkswagen’s voting shares, may have helped him hold on to his position despite increasing pressure from prosecutors.