Winterkorn and other senior figures have denied knowing about the so-called defeat devices, which can detect when a car is being tested for emissions. It allowed VW cars to emit levels of nitrogen oxide pollutants up to 40 times above what US regulations allow.
Citing documents from VW’s own investigation of the scandal, German newspaper Bild said an employee known internally as “Winterkorn’s fireman” notified superiors about the probe in a letter sent in 2014.
“A sound explanation for the dramatically increased NOx emissions cannot be given to the authorities,” the letter warned.
“It can be assumed the authorities will investigate VW systems to determine whether VW has installed a test detection in the engine control unit software.”
Winterkorn stood down last September after the scandal emerged.
“I am doing this in the interests of the company, even though I am not aware of any wrongdoing on my part,” he said at the time.
The company’s share price plunged by around 40 per cent over the course of September. The emissions cheating software affects up to 11m vehicles worldwide.
VW has had to set aside €6.7bn (£5.2bn) for the cost of recalls and another €2bn for compensation. It is also being sued for billions of dollars by US regulators. Sales have slumped, dropping by 15 per cent in the US in January compared with the same month last year.
Germany is also planning to introduce unannounced emissions tests, the country’s transport minister Alexander Dobrindt said yesterday. “There will be controls on vehicles in the style of doping tests (for athletes),” he told Bild. “Unannounced and every year.”