"VW and Audi management have been very slow to act when it appeared that leaders of the companies were implicated", Carl Tobias, a University of Richmond law professor who has closely tracked the VW case, said in an email.
Although most of the attention to VW's emissions scandal has been focused on the US, where eight company officials have been charged, investigations are also ongoing in Germany, where the cars also failed to meet on-road emissions standards.
Stadler is the most senior company official to be detained so far since Volkswagen (VW) admitted in September 2015 to using illegal software to rig US emissions tests on diesel engines.
Last week, KBA said that up to a million Daimler cars had been found to contain illegal emissions devices. The same goes for Munich prosecutors and Stadler. Stadler was arrested early Monday at his Ingolstadt home.
The United States filed criminal charges against former VW CEO Martin Winterkorn in May, but he is unlikely to face USA authorities because Germany does not extradite its nationals to countries outside the European Union.
A spokesperson for Audi confirmed the arrest to media outlets but said the company would not comment on the substance of the investigation. Two are serving prison terms; Winterkorn and the others remained in Germany and are unlikely to be extradited. Last week, German prosecutors fined Volkswagen 1 billion euros (about $1.16 billion) for not supervising the employees who designed the testing software. Oliver Schmidt, VW's emissions compliance executive in the USA, was arrested in Florida last year, and later sentenced to seven years in prison.