BERLIN, June 11 (Xinhua) -- Audi chief executive officer Rupert Stadler and another unnamed senior manager at the Volkswagen Group subsidiary are being listed as suspects in ongoing German investigations into the "dieselgate" scandal, the Munich II state prosecution office confirmed on Monday.

According to the prosecutors, raids were already carried out at the private residences of both individuals today in an attempt to secure evidence of their involvement in diesel emissions-cheating practices.

The suspects stand accused of fraud as well as indirect false certification of diesel vehicles which had been fitted with illicit defeat devices to falsify nitrogen oxide (NOx) emission levels in test settings.

"We are cooperating fully with the state prosecution office," said an Audi statement in response to the latest development.

German investigators believe that the Ingolstadt-based luxury carmaker has sold at least 210,000 diesel vehicles with illegal emissions-cheating software in the U.S. and Europe since 2009.

The listing of the two Audi executives as suspects on Monday raises the total number of individuals who have been formally charged with criminal offenses in German investigations into the "dieselgate" scandal to 20.

The Munich II state prosecution office has been seeking to compile evidence of corporate fraud and illegal advertising practices at Audi for several months and only recently searched the company's headquarters, its Neckarsulm plant, and several private premises.

So far, the former head of Audi's engine development division is the only suspect held in police custody in Germany.

Investigations by authorities were expanded once again in May after the Federal Motor Transport Agency (KBA) discovered a previously-unknown emissions cheating technology in the newest diesel versions of the popular Audi "A6" and "A7" models.

Stadler subsequently vowed to investors at his company's annual general meeting (AGM) that Audi and the Volkswagen Group had learned their lessons and would henceforth prioritize "moral and legally-proper behavior".

Nevertheless, there has been widespread speculation in German media that the "dieselgate" scandal could ultimately cost Stadler his job as part of wider reforms announced by the Volkswagen Group.