Roughly 10 percent of the diesel cars are believed to have been sold in California.

No fines have been ordered for now, as the investigation is continuing, but the EPA decided to announce the alleged violations to inform the public and “to put VW on notice of our continuing investigation”, Giles said.

As for drivers of the cars, the EPA says, “these violations do not present a safety hazard and the cars remain legal to drive and resell”.

EPA said “the effectiveness of these vehicles” pollution emissions control devices is greatly reduced during all normal driving situations.

The Environmental Protection Agency and California issued a notice of violation of the Clean Air Act to Volkswagen Friday over a device in certain models that allows vehicles to emit pollutants without detection during normal operation, according to a statement released by the EPA.

Volkswagen VOW, -3.30% VOW3, -3.23% could be on the hook for more than billion in civil penalties plus other penalties and fees for violating the clean-air rules, based on a maximum levy of $37,500 per auto , an EPA spokesperon said.

Federal law requires manufacturers certify to the EPA that their cars meet emissions standards before they are certified for sale in the U.S. Cars with defeat devices can not be certified, the EPA said.

That switch had two modes, which VW calls “road calibration” and “dyno calibration”.

Affected vehicles include 2009 – 2015 model years Volkswagen Jetta, Beetle and Golf models with TDI engines. When asked by the agencies, VW said the differences were due to testing flaws and how the vehicles were being driven.

Volkswagen is a German vehicle maker with a squeaky clean image.

If the specter of the General Motors worldwide recall of 2.6 million cars is any indication-blamed for at least 13 deaths-Volkswagen officials need to be aboveboard with federal authorities regarding the automaker’s use of software in its cars. The pollutants are linked to a range of health problems, including asthma attacks and other respiratory diseases. When EPA and CRB demanded an explanation, Volkswagen admitted the cars contained the illegal software, the EPA said.

The recall also affects 4-cylinder Audis, and the model years 2009 to 2015, reports the New York Times (reg. req.).

A spokesman for Volkswagen could not immediately be reached Friday afternoon. The vehicles all contain a device programmed to detect when they are undergoing official emissions testing, the EPA said.

The EPA said it is “incumbent on Volkswagen to initiate the process that will fix the cars’ emissions systems”.