VW has been sued by the US Justice Department over the emissions scandal that saw the German automaker fitting software in millions of cars to cheat emissions tests.
In September 2015, following an investigation by the US Environmental Protection Agency (EPA), VW admitted fitting the so-called defeat device on 11 million cars globally.
The emissions scandal has hit Volkswagen sales worldwide.
The German car giant has put aside billions of euros to deal with the fallout.
The lawsuit, on behalf of the EPA was filed on January 4 in a federal court in Detroit, Michigan.
“The complaint alleges that nearly 600,000 diesel engine vehicles had illegal defeat devices installed that impair their emission control systems and cause emissions to exceed EPA’s standards, resulting in harmful air pollution,” the filing said.
It also alleges that VW “violated” clean air laws by selling cars that were different in design from those originally cleared for sale by the EPA.
“With today’s filing, we take an important step to protect public health by seeking to hold Volkswagen accountable for any unlawful air pollution, setting us on a path to resolution,” said assistant administrator Cynthia Giles for the EPA’s Office of Enforcement and Compliance Assurance.
“So far, recall discussions with the company have not produced an acceptable way forward. These discussions will continue in parallel with the federal court action.”
The department said the filing was just the first step in “bringing Volkswagen to justice”.
VW is also facing separate criminal charges, and a raft of class-action lawsuits filed by car owners.
The EPA says that VW fitted many of its cars with a device that was able to recognize test conditions and adjust the engine settings accordingly, with the express purpose of giving distorted readings on nitrogen oxide emissions.
VW admitted to “totally screwing up”, and there has been a shake-up in the management structure and personnel as a result. Martin Winterkorn resigned as CEO and was replaced with Matthias Muller, the former boss of Porsche.
The company is currently conducting an internal investigation that it says will “leave no stone unturned”.
VW will begin recalling millions of cars worldwide soon, and has set aside €6.7 billion to cover costs. That resulted in the company posting its first quarterly loss for 15 years, of €2.5 billion in October 2015.
With the lawsuits piling up, experts say the final costs for VW are likely to be much higher than that.