Volkswagen AG's admission that it cheated to make its diesel cars appear cleaner-burning leaves it facing billions in fines, its executives risking criminal charges and its U.S. expansion plans in tatters.

VW has admitted to systematically cheating on U.S. air pollution tests for years, the Environmental Protection Agency announced Friday in citing violations that could total $18 billion in fines. The company said it has also heard from the Justice Department, and the EPA said Justice could pursue criminal prosecution.

The German automaker has struggled to gain a foothold in the world's second-biggest car market with a strategy built in part on touting the efficiency of fun-to-drive "clean diesel" vehicles that have now been shown to be anything but.