WASHINGTON - The CEO of Volkswagen apologized Sunday and VW customers said they felt duped after the Environmental Protection Agency revealed that the German automaker skirted clean air rules by rigging emissions tests for about 500,000 diesel cars.

"I personally am deeply sorry that we have broken the trust of our customers and the public," Volkswagen chief Martin Winterkorn said in a statement. He said VW has ordered an investigation and promised that the company would cooperate with regulators.

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The EPA said Friday that VW used software that allowed its diesel cars to release fewer smog-causing pollutants during tests than in real-world driving conditions. The cars, built in the last seven years, include the Audi A3, VW Jetta, Beetle, Golf and Passat models. The agency ordered VW to fix the cars at its own expense. VW also faces fines that could add up to billions of dollars.

VW edged out Toyota to become the world's top-selling automaker the first half of 2015. But a hit to its reputation from the emissions revelations could hamper its efforts at a sales rebound in the U.S. Between 2013 and 2014, VW sales plummeted 10 percent even as overall industry sales rose 6 percent.

The influential magazine Consumer Reports almost immediately suspended its "recommended" rating from the Jetta and Passat diesels until it can get a recall repair and re-test the cars.

Volkswagen marketed the diesel-powered cars as being better for the environment. After the EPA announcement, the automaker withdrew ads for its diesel cars from youtube.com and asked dealers to stop selling 2015 diesel cars with 2.0-liter engines, according to a person familiar with the matter who requested anonymity.

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Some VW customers were furious.

Zeeshan Shah, 39, of Fulton, Maryland, said that he bought a Volkswagen Diesel Passat 2015 model in July after he totaled his Jetta two years ago.

"The selling point," for the Diesel Passat was that the technology was so good, he said. Now, Shah said he plans to bring the car back to the local dealer to have it examined and doesn't want to buy another Volkswagen. "Once they cheat you on this issue, what other issues can they cheat you on?" he said. "You don't trust them."