Volkswagen's former CEO is squarely in the sights of the US Justice Department and German prosecutors, after being indicted on charges of conspiracy and wire fraud.

He has repeatedly denied any wrongdoing but former Volkswagen AG (VW) CEO Martin Winterkorn has been formally charged by US prosecutors in Detroit over the automaker’s diesel emissions scandal in 2015, dubbed “diesel dupe”.

The ex-chief of the German giant, who resigned immediately after news of the subterfuge broke, has been charged with conspiracy to defraud the US government and customers, wire fraud, and conspiracy to violate the Clean Air Act.

“Volkswagen deceived American regulators and defrauded American consumers for years,” Matthew Schneider, US Attorney for the State of Michigan, said in a statement.

“The fact that this criminal conduct was allegedly blessed at Volkswagen’s highest levels is appalling.”

The fact that this criminal conduct was allegedly blessed at Volkswagen’s highest levels is appalling.

The indictment was made public on Thursday, but it was filed in March.

Winterkorn is believed to be residing in Germany, which makes it difficult for the US to prosecute.

However, their German counterparts are currently carrying out their own investigation, which most recently included a raid on offices of the company’s Porsche division. So, this could potentially aid those proceedings.

Winterkorn, 70, is the ninth person to be hit with US criminal charges over the $US30 billion scandal. Two have pleaded guilty and are serving time in prison.

Volkswagen compliance executive Oliver Schmidt attempted to have his sentence reduced from the maximum seven years, by accusing his superiors of coaching him to lie about the vehicles’ emissions.

Schmidt’s conviction followed the carmaker’s $4.3 billion settlement with the US Justice Department in January, 2017.

It was the first time VW had ever pleaded guilty to illegal activity, and it did so with all three counts of fraud, obstruction of justice and falsifying statements.

What Winterkorn is accused of overseeing

The engines of 11 million Audi, VW, and Porsche diesel models were found to have been embedded with computer software, known as a “defeat device”, that could sense test scenarios by monitoring speed, engine operation, air pressure and even the position of the steering wheel.

When the device ascertained that a test was being carried out, it went into a safety mode of sorts, where the engine’s output was restricted.

The engine would then return to full power and performance — where it would release pollutants up to 40 times above what is allowed in the US — once it sensed the testing had finished.