German media report that Volkswagen received warnings years ago about the use of illegal tricks to defeat emissions tests.

The automaker admitted last week that it used special software to fool U.S. emissions tests for its diesel vehicles.

German weekly Bild am Sonntag reported Sunday that VW's internal investigation has found a 2007 letter from parts supplier Bosch warning Volkswagen not to use the software during regular operation.

Weekly Frankfurter Allgemeine Sonntagszeitung reported that a Volkswagen technician raised concerns about illegal practices in connection with emissions levels in 2011. The paper also cited VW's internal investigators.

Volkswagen didn't immediately respond to a request for comment.

Some 11 million VW diesel cars built since 2008 are affected by the scandal. The company has pledged to provide a free fix.