Shares in German automaker Volkswagen plunged more than 13 percent Monday just minutes after the Frankfurt exchange opened following news that many of its diesel cars in the United States had software that intentionally falsified emissions data.
The trend continued into mid-morning trading, which showed VW down more then 20 percent.
The huge drop comes one day after Volkswagen CEO Martin Winterkorn apologized for breaking the trust of VW's customers, saying the company will fully cooperate with an external investigation the automaker has ordered.
Sales of some Volkswagen diesel cars have been halted in the U.S.
On Friday, the U.S. Environmental Protection Agency ordered Volkswagen to recall nearly 500,000 vehicles for intentionally violating clean air laws.
The EPA said VW has installed so-called "defeat device" software in its diesel-powered cars that only turn on its pollution emissions controls when the car is undergoing mandatory testing, meaning the controls are completely off during everyday driving.