The Environmental Protection Agency says nearly 500,000 Volkswagen and Audi diesel cars built in the past seven years are intentionally violating clean air standards by using software that evades EPA emissions standards.

The EPA says the cars include a device programmed to detect when the car is undergoing official emissions testing. The cars only turn on full emissions control systems during that testing. The EPA says the controls are turned off during normal driving situations.

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The EPA called the company's use of the device illegal and a threat to public health.

The EPA called on VW to fix the cars' emissions systems, but said car owners do not need to take any immediate action.

VW said in a statement it is cooperating with the investigation.