Prosecutors in the north German city of Brunswick said Friday they were investigating former Volkswagen chief executive Martin Winterkorn on suspicion of fraud over the firm's "dieselgate" emissions cheating scandal.

Investigators say they have "sufficient indications" that Winterkorn may have known earlier than he has so far admitted about the cheating, adding his and 15 other names to a growing list of people facing probes for fraud and false advertising. 

With the 16 names added Friday, there are now a total 37 people under investigation.

Winterkorn is already under investigation for suspected market manipulation related to the scandal.