The U.S. government has now sued Volkswagen over its diesel-emissions cheating.
At the request of the Environmental Protection Agency (EPA), the U.S. Department of Justice filed a civil suit against Volkswagen in a Federal court in Detroit Monday.
The suit alleges VW's use of "defeat device" software in diesel cars constitutes a violation of the Clean Air Act.
VW admitted to installing the software late last year after the EPA revealed the findings of independent researchers confirming its existence at the end of September.
The software allowed cars to detect the conditions of a laboratory emissions test, and temporarily lower emissions to legal levels.
But in real-world driving, these levels were ignored. Some models emitted up to 35 times the legally-permitted amounts of nitrogen oxides, according to the EPA.
2009 Volkswagen Jetta TDI
Under the Clean Air Act, Volkswagen could be fined up to $32,500 for violations before January 2009 and $37,500 per vehicle for later violations., according to the Financial Times (subscription required).
Added together, the penalties could top $19 billion.
Volkswagen has not yet faced any criminal charges, and none of the company's executives are targeted in this civil suit.
The Justice Department said this is just the first step, however, and does not preclude future criminal charges or the prosecution of individual executives.
In a statement, VW said it "will continue to work cooperatively with the EPA on developing remedies" to address the affected diesel cars.
The automaker also said it will put in place "an independent, fair, and swift process for resolving private consumer claims related to these issues."
2014 Volkswagen Beetle TDI
Last month, Volkswagen retained attorney Kenneth R. Feinberg to administer an independent claims resolution program for TDI owners.
The claims program will allow VW to address some customer grievances out of court.
There are now more than 450 customer lawsuits pending against VW. The majority will be consolidated and heard in a court in California in the coming months.
Yet Volkswagen still hasn't confirmed when it will begin a recall of the affected TDI models.
The carmaker submitted a proposed fix for 2.0-liter models to the EPA and the California Air Resources Board in late November.
But neither agency has approved the proposal, and Volkswagen won't discuss its details in the meantime.