The US government has just announced that it has filed suit against Volkswagen (as well as its luxury brands Audi and Porsche) for alleged violating the clean air act. This response is the first official response from the govenment since it was revealed that hundreds of thousands of diesel-powered Volkswagen cars in the US (as well as millions more around) contained software that reduced emissions when in an emissions-testing mode -- but when in normal operation, they produced nitrous oxide gases at over 40 times the claimed rate.
The official suit today alleges that 600,000 diesel engines in Volkswagen's cars had this "defeat" software installed -- furthermore, the complaint claims that VW violated the clean air act by selling, introducing into commerce, or importing into the United States motor vehicles that are designed differently from what Volkswagen had stated in applications for certification to EPA and the California Air Resources Board."
It's not clear yet what specific damages the US government is looking for yet with this lawsuit; the announcement says that the government is seeking "injunctive relief and the assessment of civil penalties," which probably means massive fines are afoot. Furthermore, the government notes that this civil suit doesn't preclude other legal remedies -- indeed, the investigation is still ongoing at this point.