Prosecutors are exploring accusations that the executive may have been aware of the 'defeat device' earlier than he claimed.

Former Volkswagen chief executive Martin Winterkorn is reportedly facing a deeper investigation in Germany as prosecutors explore allegations of fraud.

Testifying in front of a German parliamentary inquiry, Winterkorn recently claimed he had no advanced knowledge of the 'defeat device' that sparked an unprecedented scandal.

VW early last year appeared to acknowledge that details of the emissions dispute were included in a May 2014 memo distributed to Winterkorn. The company downplayed the revelation, noting that it was included in "extensive weekend mail" and may not have been read by the chief executive.

The Braunschweig prosecutor's office today released a statement naming Winterkorn as one of 37 individuals accused of wrongdoing.

"On the basis of the local prosecutorial investigations, in particular the findings of the witnesses and the accused, as well as the analysis of confiscated files, there have been sufficient real indications that the accused person [Winterkorn] had been aware of the manipulating software and its effect earlier than he had publicly claimed," the agency said in a statement (Google Translate from German).

Authorities will now spend several weeks to evaluate evidence related to the expanded investigation. Any findings could impact ongoing investigations beyond Germany's borders.