Authorities in the German state of Lower Saxony have opened a criminal investigation into Martin Winterkorn, the former head of Volkswagen who stepped down last week.

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In a statement released overnight, the public prosecutor's office in Lower Saxony announced that its economic crimes unit has begun an investigation into Winterkorn.

Last week, Volkswagen filed a complaint, essentially against itself, with the prosecutor's office in Lower Saxony. The state of Lower Saxony holds a 20 percent stake in the Volkswagen Group.

One of the key matters that the investigative team will be looking into is who was responsible for giving the go-ahead to install defeat devices on vehicles powered by EA189 diesel engines, allowing equipped cars to detect and pass lab testing for emissions output.

The company's use of this sophisticated cheating software was revealed in a report, published by West Virginia University in 2014.

Two weeks ago, Volkswagen admitted to the US Environmental Protection Agency (EPA) and California Air Resources Board (CARB) that it had installed these devices on 480,000 Volkswagen Jetta, Beetle, Passat, Golf, and Audi A3 models sold in the US.

A day after Volkswagen said that up to 11 million vehicles with EA189 motors had the test cheating software installed, Martin Winterkorn, the company's CEO since 2007, announced his resignation. Winterkorn stated that he was doing so "in the interests of the company even though I am not aware of any wrong doing on my part.

By the end of the week, Matthias Muller, CEO of Porsche, had been drafted in to the company's top spot and pledged immediately to implement "even stricter governance and compliance standards", as well as "win back the trust of our customers, our colleagues, our partners, our investors and the public at large".