The head of Volkswagen’s luxury car division Audi has been questioned by investigators looking into the company’s emissions scandal, VW Chief Executive Matthias Mueller said on Tuesday.

Mueller told reporters at an event in Hanover that Audi boss Rupert Stadler had been questioned but declined to give details.

Earlier, Spiegel magazine reported that experts from U.S. law firm Jones Day, which was tasked by Audi last year to investigate the scandal, would question Stadler about when he found out about the use of cheat software.

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Earlier this week, Bild am Sonntag reported that Audi’s head of development Stefan Knirsch would be suspended as part of the investigation into the emissions scandal.

Audi has admitted its 3.0 liter V6 diesel engine was fitted with emissions-control software, deemed as illegal in the United States where the scandal has engulfed the German carmaker.

Knirsch, Audi’s former head of engine development, replaced Ulrich Hackenberg, the top engineer at Audi audvf and the VW group vlkpf , last year.

Hackenberg quit after being suspended together with two other executives closely associated with the development of the VW engine at the centre of the scandal, codenamed EA 189.