FRANKFURT — The potential damage to Volkswagen from an emissions cheating scandal grew again on Monday, as prosecutors said they were investigating the former chief executive, and the company disclosed that more than two million Audi-brand cars — a crucial source of Volkswagen’s profit — were among the affected vehicles.

German prosecutors said on Monday that they had begun an investigation of Martin Winterkorn, who until he resigned last week was the chief executive of Volkswagen. The inquiry is based on complaints from unidentified people asserting that Mr. Winterkorn should be held responsible for a widening scandal linked to diesel emissions.

Also on Monday, Volkswagen’s Audi unit said that about 2.1 million of its vehicles, almost all of them in Europe, contained software that could be used to trick emissions testers.

The Audis are among the 11 million vehicles that Volkswagen has said contained the deceptive software. But until this disclosure it was not known how broadly Audis were implicated.