Volkswagen ignored warnings from staff and a supplier years ago about the use of illegal tricks to defeat emissions tests, German media reported Sunday.
The German weekly Bild am Sonntag reported that Volkswagen’s internal investigation found a 2007 letter from parts supplier Bosch warning VW not to use the software during regular operation.
Separately, the Frankfurter Allgemeine Sonntagszeitung reported that a Volkswagen technician raised concerns about illegal practices in connection with emissions levels in 2011. The weekly also cited VW's internal investigators.
The German automaker sparked global outrage last week when it admitted that 11 million of its diesel cars built since 2008 are fitted with so-called defeat devices that activate pollution controls during tests but covertly turn them off when the car is on the road.
A VW spokesman declined to comment on the reports, saying that as a matter of principle the company wouldn't speak about what he called "rumors and speculation."
German authorities meanwhile increased pressure on the corporate giant, demanding it set out a timeline by October 7 on how it will ensure its diesel cars meet national emission standards without using the cheat technology.
The spiraling scandal has badly tarnished VW's name, left it exposed to up to $18 billion in US fines, and wiped out a third off its stock market value in a week.