German prosecutors said for the first time on Friday that they had evidence that Volkswagen’s former chief executive took part in the company’s emissions fraud, significantly raising the stakes for the carmaker and undercutting its attempts to put the scandal behind it.

Martin Winterkorn, who resigned as chief executive in September 2015 after the emissions cheating came to light, is under investigation for fraud and false advertising, according to prosecutors in Braunschweig, Germany. The authorities also increased the number of people under investigation and portrayed a far more extensive conspiracy than before.

Mr. Winterkorn and Hans Dieter Pötsch, the Volkswagen supervisory board chairman, were already under investigation for violations of securities laws. But the new fraud allegations suggest that prosecutors suspect Mr. Winterkorn had a more active role than he or the company have acknowledged.

The new allegations also point to a larger number of conspirators, and they further undermine Volkswagen’s attempts to portray the fraud, in which its diesel vehicles emitted lower levels of pollutants in lab testing than in the real world, as the work of executives and engineers below the level of its board. The assertions leave the automaker even more vulnerable to lawsuits by shareholders in the United States and Europe, who are seeking billions of dollars in damages.