Investigators eyeing Volkswagen's scandal have found no suspicious facts against the head of luxury car division Audi, Reuters reported, citing three unnamed sources.

VW chief executive Matthias Mueller said on Tuesday Audi CEO Rupert Stadler had been questioned by US law firm Jones Day but gave no details about the inquiry.

Audi has admitted its three-litre V6 diesel engine was fitted with emissions-control software, deemed as illegal in the United States where VW's diesel emissions test-rigging scandal broke a year ago.

"So far nothing has been relayed to the company that would suggest to dismiss Stadler," said one of two sources "close to Audi" told Reuters.

The questioning by Jones Day, tasked by VW last year to investigate the manipulations, has yielded no solid evidence against Stadler, a second source "close to Audi" told the news agency.

VW and Audi declined to comment to Reuters.

VW's supervisory board, meeting in Wolfsburg on Friday, also discussed the situation with Stadler, one source told Reuters.

Stadler headed Audi since 2007 and three years later joined VW group's nine-member management board.

Last Sunday, German newspaper Bild am Sonntag, citing sources, reported development chief Stefan Knirsch would be suspended this week

Investigations by Jones Day had shown that Knirsch, also an Audi board member, knew about the use of cheat software in three-litre diesel engines and gave a false promise under oath, the paper said, according to Reuters. Knirsch had already been asked to clear his desk, it added.

There has been no further news of that this week.