U.S. environmental regulators on Monday said Volkswagen AG had installed emissions-control cheats in diesel Porsche and Audi vehicles in model years 2014 through 2016.
The move on Monday expands regulatory scrutiny to Volkswagen luxury brands Porsche and Audi. The diesel models include Porsche Cayenne and Audi Quattro.
Volkswagen admitted in September it had installed software, known as defeat devices, in 11 million cars around the world that can evade emissions tests.
The emissions cheats allowed the vehicles to pass standards meant to curb nitrous oxides - a smog-causing pollutant.
The "defeat devices bypass, defeat or render inoperative elements of the vehicles' emissions control system," the Environmental Protection Agency said in a letter to manufacturers.
The EPA says the German automaker installed the cheat software on 2014 to 2016 vehicles with 3-Liter six-cylinder diesel engines. The automaker previous acknowledged rigging emissions tests for four-cylinder diesel engines.
The agency says the software has a timer that makes the cars perform differently when being tested than they perform on the road. The EPA says the cars give off up to nine times more nitrogen oxide pollution while being driven than they do when tested.
The violations cover about 10,000 vehicles, including the 2014 Touareg, 2015 Porsche Cayenne and the 2016 Audi A6 Quattro, A7 Quattro, A8 and Q5.