Investigators said the probe would focus on accusations of fraud by selling cars fitted with a software program designed to defeat regulatory emissions tests.
"The goal of the investigation is especially to clear up who is responsible," a spokesman for the prosecutors' office in Brunswick said.
VW Group had previously submitted a report of the crime to the authorities, without naming anyone as a suspect.
But Winterkorn may be particularly at risk after revelations over the weekend that VW ignored warnings from component suppliers Bosch and from its own staff about the use of the "defeat device" in its engines.
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