The US Environmental Protection Agency has told Volkswagen it is expanding its investigation into the emissions scandal.
According to the regulator, the German carmaker also installed defeat devices in 2014 to 2016 models with three-litre engines.
The EPA further claims that Porsche diesel cars were also fitted with the equipment, which cheats emissions tests.
In a statement, the agency said such devices increased emissions of nitrous oxide by up to nine times beyond the EPA's standard.
Diesel versions of the 2014 VW Touareg, 2015 Porsche Cayenne and the 2016 Audi A6 Quattro are among those included in the widened investigation.
Approximately 10,000 diesel cars already sold in the US are believed to be affected by this latest notice of violation.
With Volkswagen facing fines of $37,500 (€34,000) for every vehicle, the widened investigation could result in a further $375m (€340m) being added onto penalties already expected to comfortably enter the billions.
An EPA spokeswoman said: "VW has once again failed its obligation to comply with the law that protects clean air for all Americans."
In September, Volkswagen admitted that defeat devices had been installed in 11 million cars around the world - and the so-called "diesel issue" led to the motoring giant posting a third-quarter operating loss of €3.5bn.
After the figures emerged last week, chief executive Matthias Mueller said: ""We will do everything in our power to win back the trust we have lost."
The company has set aside €6.7bn to deal with the controversy, and indicated that this sum would rise.
This sum does not account for any penalties, fines or compensation for which the firm might be liable.
So far, 508,000 Volkswagen cars, 390,000 Audis, 132,000 Skodas, 80,000 VW commercial vehicles and 77,000 Seats have been affected by the emissions scandal in the UK - and VW is preparing to recall and fix them.
Although the company has started writing letters to those customers affected, drivers have not been told when their vehicles will be recalled. VW has said the process is expected to begin in the new year.
