Amid an emissions cheating scandal, U.S. environmental regulators told Volkswagen AG that they are expanding their investigation to Porsche, Audi and other cars from model years 2014 to 2016, Reuters reported.  The Clean Air Act violation notice issued Monday applies to roughly 10,000 diesel passenger cars sold in the U.S. since model year 2014, as well as an unknown number of 2016 models.  

In a letter posted on its website, the U.S. Environmental Protection Agency said it has determined that certain Audi, Porsche and VW models with 3.0 liter engines were rigged to pass pollution tests. The 3.0-liter vehicles spew emissions of nitrogen oxide up to nine times the EPA standard, the agency said, according to the Los Angeles Times. The affected models include the 2014 VW Touareg, the 2015 Porsche Cayenne and the 2016 Audi A6 Quattro, A7 Quattro, A8, A8L and Q5. 

“It seems extraordinary that this was not flagged within the company, investigated and declared in light of the challenges which the company has faced in the last two months,” Arndt Ellinghorst, a London-based analyst at Evercore ISI, said in a note, Bloomberg reported.

RTS3KNR Volkswagen North America CEO Michael Horn is sworn in prior to testifying before the House Energy and Commerce Committee on Capitol Hill in Washington Oct. 8. Photo: Reuters

Volkswagen admitted in September that it had installed software -- known as defeat devices -- in 11 million cars that enabled those cars to evade U.S. emissions tests, prompting an investigation from the EPA and the California Air Resources Board. The scandal led to the resignation of Martin Winterkorn, the company's chief executive, who was replaced with the top manager of its Porsche subsidiary Matthias Mueller.  

As California and federal environmental investigators continue their investigation and test diesels from other manufacturers, the number of affected vehicles could increase. 

"Today, we’re requiring VW Group to address these issues," said Richard Corey, executive officer of the California Air Resources Board, the Los Angeles Times reported. "This is a very serious public health matter. We're continuing to conduct a rigorous investigation of more vehicles until all the facts are out in the open."