"Winterkorn learned a few days ago of the existence of more detailed testimony by the former chairman, Prof. Dr. F. Piech," a lawyer for Winterkorn said in an emailed statement, adding that Winterkorn did not know any details about what Piech told prosecutors.

The prosecutor's office was not reachable for comment on Friday.

Piech, the carmaker's former chief executive and chairman who spearheaded its global expansion, quit in April 2015 after more than two decades at VW's helm following a power struggle with then-CEO Winterkorn.

Prosecutors in Braunschweig near VW's Wolfsburg headquarters said last week they were investigating Winterkorn over suspicions of fraud, citing indications that Winterkorn may have known about VW's cheating on emissions tests sooner than he has said publicly.

(Reporting by Jan Schwartz and Maria Sheahan; Editing by Georgina Prodhan)