On Monday, Rupert Stadler, the head of Volkswagen Group's Audi unit, was arrested in Germany, marking the first arrest of a VW Group high-ranking official in connection with the diesel scandal that became public in 2015.

Stadler, who took Audi's helm in 2007, had his house raided by German investigators last week. The raid occurred around the same time that VW Group agreed to pay a €1 billion ($1.2 billion) fine for "inadequate oversight" in its powertrain department.

According to local paper Sueddeuschte Zeitung as reported by Bloomberg, the arrest was made after law enforcement tapped Stadler's phone. The executive was taken into custody because prosecutors thought he might tamper with evidence. "Last week, authorities raided his house and named him a suspect in their probe of fraud and falsifying public documents in relation to selling diesel cars in Europe," Bloomberg reported.

Stadler has not been charged with anything yet, and his lawyers told Bloomberg that he is willing to answer questions from the prosecution. Stadler could be released quickly if he's cooperative, or he could be held for as long as three months, which could be extended if it's determined that he could influence witnesses.

Between 2007 and 2015, Volkswagen, Audi, and Porsche diesel vehicles were sold in the US and internationally with illegal software that allowed the cars to turn on functions of the emissions control system when the cars were being tested in a lab. The software would turn off these various functions when the car was being driven under normal conditions, giving customers better performance at the expense of much higher nitrogen oxide emissions.

In May, former Volkswagen CEO Martin Winterkorn was indicted in the US, though he remains in Germany. Five other VW Group executives have been indicted in the US: Richard Dorenkamp, Bernd Gottweis, Jens Hadler, Heinz-Jakob Neusser, and Jürgen Peter. A fifth, Oliver Schmidt, was sentenced to seven years in prison last year. One former engineer, James Liang, is currently serving 40 months in prison in the US.

In Germany, investigations are underway from three different municipalities: Munich, Stuttgart, and Braunschweig. "The Munich probe now involves 20 people linked to Audi," Bloomberg notes. Indeed, researchers have found evidence that emissions cheating at VW Group started as early as 2004 at Audi.