BERLIN (AP) — Prosecutors in Germany say they have sufficient evidence to indicate that former Volkswagen CEO Martin Winterkorn may have known of his company's emissions cheating software earlier than he claims.

The 69-year-old stepped down in September 2015, days after news of Volkswagen's use of software to cheat on emissions emerged in the U.S., saying at the time that he was not aware of any wrongdoing on his part.

Prosecutors in Braunschweig said in a statement Friday that they have increased the number of suspects in their investigation of VW from 21 to 37.

Winterkorn and others are being investigated on allegations they committed fraud by failing to inform shareholders early enough of the problems the company could face.