FILE -  The Volkswagen logo on the hood of a 2012 Beetle at a Volkswagen dealership.
FILE - The Volkswagen logo on the hood of a 2012 Beetle at a Volkswagen dealership.

Shares in German automaker Volkswagen plunged more than 13 percent Monday just minutes after the Frankfurt exchange opened following news that many of its diesel cars in the United States had software that intentionally falsified emissions data.

The trend continued into mid-morning trading, which showed VW down more than 20 percent.

The huge drop comes one day after Volkswagen CEO Martin Winterkorn apologized for breaking the trust of VW's customers, saying the company will fully cooperate with an external investigation the automaker has ordered.

Sales of some Volkswagen diesel cars have been halted in the U.S.

On Friday, the U.S. Environmental Protection Agency ordered Volkswagen to recall nearly 500,000 vehicles for intentionally violating clean air laws.

The EPA said VW has installed so-called "defeat device" software in its diesel-powered cars that only turn on its pollution emissions controls when the car is undergoing mandatory testing, meaning the controls are completely off during everyday driving.  

The agency says the cars emit nitrogen oxide up to 40 times above the limits allowed under the federal Clean Air Act.  Nitrogen oxide is a major component of smog and has been linked to asthma and other respiratory illnesses.

"We expected better from Volkswagen," said Cynthia Giles, the EPA's assistant administrator for enforcement. Environmental regulators in California have issued a separate notice to Volkswagen, and is joining the EPA and the U.S. Justice Department in an investigation of the charges.  

The cars, sold in the U.S. between 2009 and 2015, include such popular models as the Jetta, Beetle, Passat and the Audi A3.  The EPA has ordered Volkswagen to fix the cars at its own expense, and could impose an $18 billion fine on the company, which equals $37,500 for each recalled vehicle.  

Frank O'Donnell, the director of Clean Air Watch, called the charges against the automaker "truly appalling," and said it was "cheating not just car buyers but the breathing public."