The U.S. Environmental Protection Agency accused Volkswagen of intentionally dodging air-pollution rules on almost 500,000 cars sold in the USA since 2008.

The agency alleged the automaker installed software on nearly half a million diesel passenger cars that allowed it to cheat emissions tests. The software, dubbed a “defeat device”, activates full emissions controls only during testing but reduces their effectiveness while driving. The result is that the vehicles emit nitrogen oxides up to 40 times the acceptable standard, potentially exposing people to harmful pollutants which increase the risk of respiratory conditions.

Volkswagen CEO Martin Winterkorn, who was in charge of the company at the time of the cheating, apologized to customers on Sunday for breaking their trust and promised an internal investigation.

Volkswagen AG may be forced to recall 482,000 cars with four-cylinder diesel engines. EPA ordered the German automaker to fix the cars’ emission systems at its own expense, officials said, although the process could take up to a year.

Volkswagen also faces billions of dollars in fines after U.S. officials said the automaker violated two parts of the federal Clean Air Act, although the exact amounts of vehicles were not determined. The penalties will be up to $37,500 per car which is more than $18 billion.

The EPA is working with the Justice Department and an investigation is underway, offiscials announced. The German automaker said in a statement that it is “cooperating with the investigation” but declined further comment.

The automaker, Europe’s largest by sales, has already issued a stop-sale for 2015 and 2016 models, Volkswagen spokesman Mario Guerreiro said Sunday and apparently, this is the first step before a massive recall.

The affected vehicles, all built in the last seven years, include Audi A3 and Volkswagen Jetta, Beetle, Golf and Passat with model years between 2009 and 2015.

The automaker’s apparent motivations for cheating the emissions tests remain unclear. According to an EPA spokeswoman, it would be “premature to speculate” on why Volkswagen did this.