BERLIN — German media reported Sunday that Volkswagen was warned years ago about the use of illegal tricks to defeat emissions tests. The German automaker admitted last week that it used special software to fool U.S. 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 that 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 wouldn't comment on what he called "rumours and speculation." "Volkswagen is working with all its strength to conduct a thorough and merciless investigation of this matter," Andreas Lampersbach said in an email. 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. "Afterward we will provide a timetable to modify the vehicles of affected customers. This will take several weeks."