The Environmental Protection Agency on Monday issued a second notice of violation against Volkswagen, and this time included Porsche and Audi models.
Monday's announcement adds more vehicles to the list of affected cars, including the 2014 VW Touareg, the 2015 Porsche Cayenne and the 2016 Audi A6 Quattro, A7 Quattro, A8, A8L and Q5.
The EPA said the German automaker installed software designed to defeat emissions tests on 2014 to 2016 vehicles with 3-liter, six-cylinder diesel engines. In September, the automaker acknowledged rigging emissions tests for four-cylinder diesel engines.
The new notice marks the first time a Porsche model has been implicated in the scandal.
Volkswagen refuted the allegations and said it had installed no software on its 3-litre V6 diesel motors to improperly change emissions values. It said that it was cooperating fully with the EPA.
Porsche, which is owned by The Volkswagen Group, said on Monday that it will also cooperate fully with the authorities.
"We are surprised to learn this information. Until this notice, all of our information was that the Porsche Cayenne Diesel is fully compliant," it said in a statement.
"VW has once again failed its obligation to comply with the law that protects clean air for all Americans," said Cynthia Giles, assistant administrator for the Office for EPA's Enforcement and Compliance Assurance.
"All companies should be playing by the same rules. EPA, with our state, and federal partners, will continue to investigate these serious matters, to secure the benefits of the Clean Air Act, ensure a level playing field for responsible businesses, and to ensure consumers get the environmental performance they expect."
The second notice adds 10,000 vehicles to the list of affected VW cars, which could mean more fines for the German automaker. Previously, the automaker said 11 million cars worldwide, including about half a million in the United States, were affected.
The EPA and the California Air Resources Board have investigated the carmaker for alleged software programs that allow certain vehicles to violate emissions standards.
— The AP and Reuters contributed to this report.