US prosecutors filed charges against Volkswagen's former chief executive officer Martin Winterkorn in the scandal of VW cars programmed to cheat on government diesel emissions tests.

The indictment unsealed on Thursday in a Michigan federal court alleges Mr. Winterkorn conspired to defraud the US and violate federal laws designed to control air pollution.  The indictment also charged five more VW executives.

"The indictment unsealed today alleges that Volkswagen's scheme to cheat its legal requirements went all the way to the top of the company,"said US attorney general Jeff Sessions, "These are serious allegations and we’ll prosecute this case to the full extent of the law."

Volkswagen agreed to plead guilty as a corporation earlier this year.  It will pay US$4.3 Billion in fines.  Another executive, Oliver Schmidt, was convicted of fraud and sentenced to seven years in prison, and fined $400,000.  Schmidt was arrested because he left Germany and went to Miami for holiday.  Germany is unlikely to extradite any of its citizens, Winterkorn included, to face US charges in this matter.

Mr. Winterkorn has steadfastly denied any prior knowledge of the scam to install software in VW's diesel cars that made them perform "cleaner" during emissions tests than they normally did on the road.

"It is not comprehensible why I was not informed early and clearly about the measurement problems," Winterkorn told a German parliamentary investigation last year.