German prosecutors have started a probe into the former chief executive of scandal-hit Volkswagen (XETRA:VOW), according to reports on Monday.
The investigation into Martin Winterkorn looked at allegations of fraud in the sale of cars with manipulated emission data, news agency Reuters reported.
Winterkorn stepped down last week after it emerged that VW had fitted systems in 11 million diesel-powered cars to hide their actual emission levels.
VW subsidiary Audi has also admitted that 2.1m of its cars also had the software installed.
Reuters also cited German newspaper reports claiming VW technicians and one of its suppliers, Bosch, warned in 2011 and 2007 respectively about software designed to thwart emissions tests.
VW last week appointed Matthias Mueller as its new chief executive, although Mueller will stay as chief executive of the group's Porsche arm until a successor has been named.
VW, one of the best known names in the global industry, has admitted cheating diesel emission tests in the US by installing certain software in vehicles.
It said up to 11 million cars worldwide were affected and possibly subject to a global recall.
It said it was taking a €6.5bn (US$7.27bn) provision against earnings to cover the costs of fixing it.
Countries including Spain have launched investigations into the emissions test data.
VW has reportedly been told it must recall around 720,000 cars in Spain, far more than the 480,000 expected to be recalled in the US, as other countries across Europe announced inquiries.