Volkswagen announced Sunday it will suspend the sale of vehicles sold in the United States containing the diesel engines that regulators say include software meant to evade environmental restrictions. A representative said the German company will halt the sale 2015 models of Volkswagen and Audi vehicles 4-cylinder turbo diesel engines, though 2016 models will not be affected.

Volkswagen could face billions of dollars in fines from the U.S. Environmental Protection Agency for what the government says was a deliberate attempt to flout a smog reduction law by deeming the 4-cylinder turbo engines “clean diesel.” Along with the recall Sunday, chief executive Martin Winkerton apologized and promised to “cooperate fully” with the investigation.

“I personally am deeply sorry that we have broken the trust of our customers and the public,” Winkerton said in a statement.

He did not deny the accusation Volkswagen planted software in its cars capable of detecting when government emissions testing was being conducted. At all other times, the cars would emit 40 times the pollution permitted by the Clear Air Act, the Times reported.

Volkswagen did not disclose how many vehicles are affected, though the EPA previously estimated 482,000 cars are equipped with the software. The 4-cylinder turbo diesel engine is most commonly found in Volkswagen's Golf, Jetta, Passat, Beetle and the Audi A3 luxury compact.

The violations are punishable with financial penalties of up to $37,500 per car, or $18 billion, the Wall Street Journal reported.

“Using a defeat device in cars to evade clean-air standards is illegal and a threat to public health,” Cynthia Giles, assistant administrator for the EPA’s enforcement group, told the Journal. “These violations are very serious. We expected better from VW.”