U.S. investigators have detected software that may have helped Daimler AG-manufactured diesel cars pass regulatory emissions tests, Reuters and Bloomberg News reported citing an article from the Bild am Sonntag newspaper.

The German automaker developed software functions that reduce or regulate the application of AdBlue fluid that helps eliminate harmful exhaust gases, even though Daimler engineers raised concerns regarding their legality, Bild am Sonntag reported, citing confidential documents and internal e-mails.

Daimler spokesperson Joerg Howe told Bloomberg News in an email that the documents have "selectively been released in order to harm Daimler and its 290,000 employees." He also said that the company continues to fully cooperate with U.S. authorities.

"The authorities know about the documents and no complaint has been filed," Howe said.

The automaker's shares dropped 2.2% by 5:03 pm in Frankfurt , making it the worst performer in Germany's DAX index.