The US Justice Department has opened legal proceedings against Volkswagen (VW) over the scandal that saw the German car maker fit software that allowed millions of vehicles to cheat emission tests. Late on 4 January, a lawsuit claiming the beleaguered car manufacturer "violated" clean-air regulations was filed on behalf of the US Environmental Protection Agency (EPA) at 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," said the filing.

According to the lawsuit, the car maker deliberately tampered with vehicles destined for the US market by fitting them with what the US regulators described as a "defeat device", software designed to cheat emission tests.

The devices were designed to keep diesel engines' emissions in the required range during testing before switching off in real-life driving conditions, to boost performance. That resulted in diesel engines producing greenhouse gas emissions 40 times greater than the federal standard.

Under US laws, car manufacturers are required to disclose such devices to the regulators. However, since VW kept the software secret, the regulatory filings allege the company's vehicles were sold without the required "certificate of conformity", which is issued by the EPA to regulate new cars produced or imported into the US.

The Justice Department indicated the lawsuit was only first step in bringing VW to justice.

"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 Cynthia Giles, assistant administrator 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."

In September 2015, the Germany-based car giant admitted installing the software on some 11m diesel and SUV vehicles worldwide since 2009 and has since put €6.7bn (£4.6bn, $7.2bn) to front the costs of fines and reimbursements.

In the aftermath of the scandal, former chief executive Martin Winterkorn resigned and was replaced by ex Porsche boss Matthias Mueller, while in October the company posted its first quarterly loss since 2000.

Late last year, VW, the second-largest car maker in the world, told investors it had hired a US-based law firm to conduct internal investigation that would "leave no stone unturned".