A criminal prosecution has been opened in Germany against former Volkswagen chief executive Martin Winterkorn over the diesel emissions scandal.
The investigation will focus on alleged fraud and finding out who was responsible, according to a statement issued by prosecutors in Braunschweig.
A number of criminal complaints have been made in relation to the rigging of emissions tests, including one from Volkswagen itself that does not name any suspects.
Mr Winterkorn resigned last week after the scandal broke.
He issued a statement saying: "I am doing this in the interests of the company even though I am not aware of any wrongdoing on my part."
Julia Meyer, spokeswoman for the German prosecutors, described the case as "very broad". She added: "In other such investigations it has taken many months, sometimes years."
Volkswagen's upmarket brand Audi confirmed that 2.1 million of its vehicles are among those with engines affected by the defeat device software used to con testers in the US.
The engines were built into 1.6 and 2-litre turbo diesel models including the A3.
Audi has not released figures on how many of those cars were sold in the UK.
VW has admitted that 11 million vehicles worldwide were fitted with sophisticated software which conned testers into believing their vehicles met environmental standards.
The Environmental Protection Agency said 482,000 of the German car-maker's 2009-15 models in the US were fitted with the defeat device to switch engines to a cleaner mode when they are undergoing official emissions testing.
Once on the road, the cars produced nitrogen oxide pollutants at up to 40 times the legal standard.