Porsche has been implicated in the Volkswagen emisisons scandal for the first time after regulators found "defeat devices" in the luxury diesel vehicles.
The carmarker used the devices to cheat air pollution tests in mostly larger, more expensive models.
The US Environmental Protection Agency has said it is investigating 3.0 litre V6 diesel engines installed from 2014 to 2016.
It claims Volkswagen installed software on thousands of Porsche, Audi and VW cars with these larger engines so they emitted less pollution during tests.
The previous revelations of cheating involved four-cylinder diesels in smaller cars.