Reports in German media suggest carmaker Volkswagen's luxury Audi subsidiary, known for its high-tech engineering prowess, was more deeply involved in the Dieselgate emissions-cheating scandal than was previously known.

Four of Audi's top engine designers have been suspended, purportedly because they either developed illegal software for the three-liter TDI diesel engine or knew about it, public broadcasters NDR and WDR and the "Süddeutsche Zeitung" daily reported on Wednesday evening.

The software allowed engines to appear much cleaner during laboratory testing than they were in real-world use.

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Among those suspended is Audi's director of technical development, Stefan Knirsch. This comes days after the newspaper "Bild am Sonntag" reported he would be taking a leave of absence.

The new media reports cite findings by the law firm Jones Day that suggested evidence against Knirsch was "overwhelming." VW's supervisory board hired the firm to investigate the scandal. Knirsch denies the allegations.

In contrast, Jones Day has found no concrete evidence against board chairman Rupert Stadler, the reports said.

Email evidence

Investigators reportedly found numerous documents they believe prove Ingolstadt-based Audi's role in the fraud. A key piece of evidence is an email from 2007. An Audi engineer wrote to a group of the company's executives about the strict exhaust emission limits in the US.

It would not be possible for Audi cars to meet these requirements "completely without cheating," he wrote.

In the course of its investigation at VW, the reports said, Jones Day found more and more evidence suggesting Audi had been closely involved in Dieselgate. They said Audi engineers' support was so important, the company is being described within the Volkswagen Group as the "mother of the fraud."

Silence at Audi

An Audi spokesman refused to comment, citing ongoing proceedings in the US under District Court Judge Charles Breyer. He has given preliminary approval to a $15-billion settlement Volkswagen agreed to with the US authorities for cars with two-liter engines; the carmaker has until October 24 to submit a proposed solution for manipulated three-liter cars.

Volkswagen has so far not published any of Jones Day's findings. In April, the company, citing legal advice, said "a disclosure of interim results of the investigation at this point in time would present unacceptable risks for Volkswagen and, therefore, cannot take place now."

The scandal has tarnished Volkswagen's image, but has had little effect on its sales. The company, which sells 12 brands, is slightly ahead of rivals Toyota and GM as the global leader in car sales.

ul, sgb/mg (afp, dpa, BamS, SZ)