Volkswagen’s former Chief Executive Officer Martin Winterkorn was charged in a Michigan federal court with conspiracy and wire fraud in relation to a probe into the German automaker’s efforts to cheat U.S. diesel emissions testing, according to an indictment unsealed Thursday.

Winterkorn, who stepped down from his role as CEO days after the scandal was revealed, is accused of conspiring to defraud the U.S. and to violate the Clean Air Act. The charges were filed under seal in Detroit on March 14. He is the highest-ranking person to be charged in the matter.

“The indictment unsealed today alleges that Volkswagen’s scheme to cheat its legal requirements went all the way to the top of the company,” Attorney General Jeff Sessions said in a written statement. “These are serious allegations, and we will prosecute this case to the fullest extent of the law.”

Winterkorn’s lawyer in Germany, Felix Doerr, didn’t immediately reply to an e-mail seeking comment.

“Volkswagen continues to cooperate with investigations by the Department of Justice into the conduct of individuals,” the company said in an emailed statement. “It would not be appropriate to comment on individual cases.”

VW admitted in September 2015 that it outfitted about 11 million diesel cars worldwide with a defeat device, embedded software that allowed the vehicles to recognize when they were being tested in laboratory conditions, and to reduce emissions to meet acceptable levels. According to the indictment, Winterkorn was briefed on both the emissions issue and how U.S. regulators were threatening to delay certifying 2016 cars for sale, at a July meeting in Wolfsburg, Germany, where the company is based.

The carmaker pleaded guilty in January 2017 to using false statements to import cars into the U.S. and to obstructing investigations, and paid $4.3 billion in penalties.

Volkswagen’s new CEO, Herbert Diess, pledged earlier Thursday that the German manufacturer would step up compliance systems to prevent the sort of misconduct that set off the deepest crisis in the company’s history. Diess spoke for the first time in his new role as CEO at the company’s annual shareholder meeting in Berlin after VW abruptly replaced Winterkorn’s successor, Matthias Mueller, last month.

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