The prosecutor's office in the city of Braunschweig announced that the investigation would focus on "allegations of fraud in the sale of cars with manipulated emissions data."

"The aim of the investigation is to clarify the chain of responsibility," it added.

Winterkorn stepped down last Wednesday after US authorities revealed that Volkswagen had cheated in emissions tests using software installed in diesel vehicles. The software recognized when a vehicle was being tested and reduced the amount of nitrogen oxide emitted, although under road conditions the cars belched out up to 40 times as much of the ozone-causing substance as US law allows.

The scandal has spread as other countries announced they would launch probes into the diesel engines involved.

The presidium of VW's supervisory board last week announced it would itself file charges against persons unknown over "obvious irregularities" with regard to diesel engine pollution tests. Board members said they would fully support investigators in their work.

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