Audi said it was cooperating fully with investigators but declined to comment further. Volkswagen also declined to comment.
Martin Winterkorn resigned in September 2015 as chief executive of Volkswagen days after the scandal came to light. Last month, the United States Department of Justice indicted Mr. Winterkorn, the former chief executive of Volkswagen, on fraud charges in connection with the emissions deception.
But Volkswagen kept many of Mr. Winterkorn’s close associates, including Mr. Stadler. The company insisted that the wrongdoing was the work of a small group of engineers and has been reluctant to clean house.
Hans Dieter Pötsch, the Volkswagen chief financial officer throughout the time the software was in use, is now chairman of the company’s supervisory board. Prosecutors have said Mr. Pötsch is under investigation for failing to fulfill his duty to inform shareholders of the risks Volkswagen was taking, but have not accused him of taking part in the diesel fraud.
Mr. Winterkorn’s replacement in 2015 was Matthias Müller, another longtime insider who worked under Mr. Stadler at Audi. Mr. Müller resigned under pressure in April in part because he was having trouble moving Volkswagen beyond the scandal.
The new chief executive, Herbert Diess, is a former BMW manager who joined Volkswagen only a few months before United States regulators discovered the cheating. By Volkswagen standards Mr. Diess is an outsider, but it is probably too early to judge whether he will be able to shake the stigma of the scandal more effectively than his predecessor.