German prosecutors opened an investigation Monday into former Volkswagen AG CEO Martin Winterkorn after the company admitted to cheating on emissions tests, Bloomberg reported. The news comes as the automaker has sacked several senior executives amid a growing scandal that has seen its shares plummet more than 25 percent since last week.

Volkswagen has been accused of using deceptive software to deceive regulators into believing their diesel cars were compliant with emission regulations. The company has admitted to rigging some 11 million cars, which were believed to use pollutants considerably greater than allowed per U.S. standards.

Volkswagen’s luxury brand, Audi, said in a statement Monday that 2.1 million of its cars were also affected by the emissions scandal, as some 1.42 million of its cars in Western Europe were fitted with software allowing the manufacturer to cheat U.S. emissions tests, Audi said Monday .

Prosecutors are hoping to determine where the responsibility lies for the alleged manipulated emission tests, prosecutors in Braunschweig, Germany, said Monday in an email statement. 

Winterkorn resigned as CEO of the giant automaker Sept. 23. He was replaced by the former Porsche unit chief, Matthias Mueller. VW now faces at least $18 billion in fines in the U.S. alone.

Winterkorn has denied involvement in or knowledge of any wrongdoing.

At least 27 state attorneys general in the U.S. have also announced plans last week to open a multi-state investigation into the company. Volkswagen told U.S. dealers to halt sales of some of its 2015 diesel cars, as Winterkorn said in a letter Sunday, "I personally am deeply sorry that we have broken the trust of our customers.”

The company is likely to face a difficult time recovering its reputation, analysts warned. "It is clearly a massive blow to the reputation of the company," Christian Ludwig, automotive analyst at Bankhaus Lampe, said on CNBC, talking about the Volkswagen scandal.