Volkswagen (VW) is reportedly the target of a criminal investigation on emissions by the US justice department according to a Bloomberg report.
The company's shares closed down 23 per cent today, after chief executive Martin Winterkorn apologised for manipulating the carbon emissions data from Volkswagen and Audi vehicles.
"I personally am deeply sorry that we have broken the trust of our customers and the public," he said in a statement yesterday.
"We will cooperate fully with the responsible agencies, with transparency and urgency, to clearly, openly, and completely establish all of the facts of this case. Volkswagen has ordered an external investigation of this matter."
On Friday, the US Environmental Protection Agency (EPA) ordered the company to withdraw nearly half a million cars and claimed that it had deliberately installed a device to mislead emissions testers. VW could face fines of up to $37,500 (£24,000) per vehicle, according to the EPA.
The total fine could be more than $18bn, which would wipe out nearly all VW's cash balance, though analysts have said the actual figure is likely to be much lower.
The EPA claimed the device is programmed to detect when the car is undergoing emissions test, during which time it will turn on its full emissions control systems. These controls are reportedly then turned off when the car is driving in “normal” situations, when the cars pollute more heavily than reported by Volkswagen.