German prosecutors have begun an investigation against former Volkswagen chief executive Martin Winterkorn.
The probe will look at "allegations of fraud in the sale of cars with manipulated emissions data", the prosecutor's office said.
Mr Winterkorn quit last week after almost nine years at the helm of VW, saying he had no knowledge of the manipulation of emissions results.
Regulators in the US had found "cheat" software in some diesel engines.
In the German legal system, anyone can file a criminal complaint with prosecutors, who are then obliged to examine them and decide whether there is enough evidence to open a formal investigation.