Investigators in the US enquiring into Daimler's Mercedes-Benz brand found cars had software which may have help them to pass diesel emissions tests, weekend media reports said.

Reuters cited a Bild am Sonntag newspaper report which said confidential documents showed the investigators had found several software functions that helped Daimler cars pass emissions tests, including one which switched off emissions cleaning after 26km of driving.

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Another function allowed the emissions control system to determine if the car was being tested based on speed or acceleration patterns.

Bild am Sonntag also cited emails from Daimler engineers questioning whether those software functions were legal.

A Daimler spokesman declined to comment to Reuters on the content of the documents but said the automaker was cooperating fully with US authorities and had agreed upon strict confidentiality with the Department of Justice.

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

"The documents available to Bild have obviously selectively been released in order to harm Daimler and its 290,000 employees."

Daimler is facing ongoing investigations by US and German authorities into excess diesel emissions and has acknowledged investigations could lead to significant penalties and recalls, Reuters noted.

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