The former of chief of Volkswagen's supervisory board has allegedly told German investigators that ex-CEO Martin Winterkorn knew of the firm's emissions cheating systemswell before the dieselgate scandal went public.

A report published Friday in German weekly "Der Spiegel" reported that former board chief Ferdinand Piech "has incriminated the group's former chief executive Martin Winterkorn with a detailed statement to prosecutors."

Winterkorn has maintained that he knew nothing of the dieselgate emissions scam before it became public in September 2015, comments he echoed before a parliamentary inquiry last week.

The former chief still resigned days after VW admitted that it had installed so-called defeat devices in some 11 million diesel engines, making their cars appear less polluting than they were.

Winterkorn told the parliamentary committee that "total clarity was and is the order of the day," and that he was still trying to understand how the scandal could have happened.

However, Friday's report claims that Piech had told prosecutors in the German city of Brunswick that he himself had learned from an informant that US authorities were investigating VW's software manipulation systems as early as February 2015, and that US officials had even passed their findings on to the German car giant.

According to the "Spiegel" article, when Piech approached Winterkorn about the cheating system, the then-CEO reportedly assured him that no such document from the US authorities existed.

Winterkorn facing fresh inquiry

Last week, prosecutors in the German city of Brunswick announced they were investigating Winterkorn for fraud and market manipulation, saying they had "sufficient indications" that the former chief knew of the cheating. Authorities have not confirmed whether that investigation is linked to the "Spiegel" report and Piech's alleged testimony.

Felix Doerr, a lawyer for Winterkorn, told news agency AFP in a statement that his client was made aware that Piech had spoken to authorities a few days ago but did not know "the details of the statement."

"Mr Winterkorn will respond to the allegations against him, and therefore also this statement, as soon as the documents from the Brunswick prosecutor's office are made available to him," Doerr said.

dm/gsw (AFP, dpa, Der Spiegel)