The EPA today accused Volkswagen AG of a second series of emissions violations that include 10,000 U.S. passenger vehicles whose model years date from 2014 through 2016.

The vehicles covered by today's notice of violation (NOV) of the Clean Air Act (CAA) are the 3.0-liter diesel engine versions of the 2014 VW Touareg, the 2015 Porsche Cayenne and the 2016 Audi A6 Quattro, A7 Quattro, A8, A8L and Q5.

The most recent vehicles in violation of EPA emission standards were emitting up to nine times the allowed levels of nitrous oxide (NOx).

NOx pollution contributes to harmful ground-level ozone and fine particulate matter. Exposure to these pollutants has been linked to a range of serious health effects, including increased asthma attacks and other respiratory illnesses.

Volkswagen responded in a statement that it had received the NOV from the EPA about the V6 TDI engines, but said the complaint only targeted "a software function which had not been adequately described in the application process."

"Volkswagen AG wishes to emphasize that no software has been installed in the 3-liter V6 diesel power units to alter emissions characteristics in a forbidden manner. Volkswagen will cooperate fully with the EPA to clarify this matter in its entirety," the statement read.

The EPA initially slapped Volkswagen with an NOV on Sept. 18, after which the automaker admitted it had installed "defeat device" software on 11 million cars worldwide that allowed them to evade emissions tests. Those vehicles covered the Jetta, Beetle, Golf and Audi A3 models from 2009 to 2015 with 2.0-liter diesel engines.

The defeat device allowed the cars to meet emissions standards during tests. But after the tests were done, the vehicles could spew nitrogen oxides (NOx) at up to 40 times the level allowed by law, in violation of the Clean Air Act, according to the EPA.

The defeat device worked by sensing when a vehicle was undergoing a federal emissions test procedure. It would then operate in a low NOx "temperature conditioning" mode. In that mode, a vehicle meets emission standards.

"At exactly one second after the completion of the initial phases of the standard test procedure, the vehicle immediately changes a number of operating parameters that increase NOx emissions and indicates in the software that it is transitioning to 'normal mode,' where emissions of NOx increase up to nine times the EPA standard, depending on the vehicle and type of driving conditions," the EPA stated in its latest NOV.

Volkswagen AG CEO Martin Winterkorn stepped down after the scandal broke; it has so far cost the company billions in lost value as its stock price has plunged. Winterkorn stated that he was "shocked" and "not aware of any wrongdoing" on his part.

The EPA, after discovering the initial group of vehicles, initiated a second round of tests of all 2015 and 2016 light duty diesel models with 3.0-liter engine available in the U.S using updated procedures specifically designed to detect defeat devices.

That testing led directly to the violations alleged in today's NOV. The NOV is based on vehicle emission testing performed by the EPA's National Vehicle and Fuel Emissions Laboratory, CARB's Haagen-Smit Laboratory and Environment Canada's River Road Laboratory.

"VW has once again failed its obligation to comply with the law that protects clean air for all Americans," Cynthia Giles, assistant administrator for the Office for EPA's Enforcement and Compliance Assurance, said in a statement. "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."