Volkswagen is probing deeper into its diesel engine range to see whether additional vehicles contain illegal software which circumvents air emission standards.

In a statement released Thursday, the German automaker said two versions of its diesel engines, EA 288 EU5 and EU-6 models, do not have "defeat devices" installed and are therefore clean when it comes to emissions testing.

However, the defeat devices have been installed in millions of vehicles. While new diesel models offered in the European Union meet legal requirements, the investigation is ongoing -- and older models sold outside the EU may be affected.

If older diesel engines do contain the same defeat devices, this could raise the number of vehicles requiring fixes beyond 11 million. For Volkswagen, this leads to a heavier financial burden, a lengthening of the investigation and potentially additional fines levied by governments and damages pursued by customers.

First disclosed by the Environmental Protection Agency (EPA), the US regulator claimed a number of Volkswagen and Audi vehicles used "sophisticated software algorithms" to circumvent clean air emission laws. These "defeat devices" are at the center of the scandal which has brought the Volkswagen brand to its knees in recent months.

The defeat devices ensure that vehicles meet emissions standards when in laboratory and test environments, however, emissions standards are not met in typical driving conditions. While the software breaks clean air and emissions regulations across Europe and the US, Volkswagen is quick to emphasize that affected models are still roadworthy and safe to drive.

Earlier this month, Volkswagen said the German Federal Motor Transport Authority (KBA) enforced a recall of Volkswagen Group vehicles in the country. According to the KBA, the automaker will need to recall roughly 2.4 million vehicles in Germany as part of a wider European recall of 8.5 million vehicles.

These figures are in addition to recalls outside of the region. In the United States, almost 500,000 cars are to returned to dealerships for repairs.

According to The Financial Times, 10 senior executives have been suspended as part of its investigation into the emissions software problems. The suspended executives reportedly include Richard Dorenkamp, former head of VW's technical development for low emission engines, Falko Rudolph, the former head of diesel engine development and Ulrich Hackenberg, an Audi board member.

Earlier this month, Volkswagen's new CEO Matthias Muller -- replacing Martin Winterkorn who resigned in September -- promised employees "swift and relentless clarification" of the emissions scandal.

Muller said there was "no excuse" for the Volkswagen's Group's failure when it came to the defeat devices, which "went against everything its people stand for." In a speech designed to boost the morale of employees caught up in the scandal, Muller said:

"We can and we will overcome this crisis, because Volkswagen is a group with a strong foundation. And above all because we have the best automobile team anyone could wish for."

"Apart from the enormous financial damage which it is still not possible to quantify as of today, this crisis is first and foremost a crisis of confidence," he said. "That is because it is about the very core of our company and our identity: it is about our vehicles. Our most important task will therefore be to win back the trust we have lost -- with our customers, partners, investors and the general public."

Muller admitted the financial damage caused by the scandal cannot yet be quantified. Volkswagen has set aside $7.3 billion, but this won't cover more than repairs and a massive PR effort to try and retain a customer base. On top of damages, repairs and recalls, Volkswagen must also face up to fines issued by governments where the vehicles have failed to meet emissions standards, as well as class-action lawsuits already levied against the automaker for misleading customers.

If you believe your vehicle might be affected, you can enter your car's number at Volkswagen's website to see if a recall to a dealership to have the software fixed is likely.

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