Volkswagen was warned years ago about the use of illegal tricks to defeat emissions tests, according to reports in the German media.
The German car maker admitted last week that it used special software to fool US emissions tests for its diesel vehicles. Some 11 million VW diesel cars built since 2008 are affected by the scandal.
German weekly Bild am Sonntag reported that VW's internal investigation has found a 2007 letter from parts supplier Bosch warning Volkswagen not to use the software during regular operation.
Separately, the Frankfurter Allgemeine Sonntagszeitung reported a Volkswagen technician raised concerns about illegal practices in connection with emissions levels in 2011. The weekly also cited VW's internal investigators.
A spokesman for Volkswagen declined to comment on the reports, saying that as a matter of principle the company would not respond to what he called "rumours and speculation".
In an email, Andreas Lampersbach said: "Volkswagen is working with all its strength to conduct a thorough and merciless investigation of this matter."
VW would hold those responsible for rigging the emissions test to account and discuss technical solutions for the problem with authorities, he said, citing a statement by the company's supervisory board.
"Afterwards we will provide a timetable to modify the vehicles of affected customers. This will take several weeks."