Former Volkswagen chief executive Martin Winterkorn might possibly have known that the company's cars were fitted with software to cheat emission test results as early as May 2014, a report says.

Citing its two sources knowledgeable about the matter, Reuters claims that a high-ranking staff member, known internally as "Winterkorn's fireman," advised senior managers of the company that United States regulators might look at VW's engine software as part of a probe into pollution levels.

Sources say the said employee warned the senior managers through a letter, which was reportedly delivered to them more than a year before the German automaker admitted that its cars are loaded with software to cheat emissions tests. Reuters says this raises "questions about how much senior managers knew about the scandal."

This purported internal letter informing VW's senior managers about the investigation was initially reported by German newspaper Bild am Sonntag. The discovered letter is said to be part of the company's internal investigation carried out by law firm Jones Day.

The letter originated from the company's product quality and safety department, according to sources. However, they tell Reuters that they're unsure whether or not Winterkorn had read it.

"It can be assumed that the authorities will investigate VW systems to establish whether Volkswagen has implemented test-recognition software," says Bild am Sonntag referring to the letter.

A spokesperson from the company declined to comment regarding the matter and the representatives from Winterkorn were not available to give comments. The VW spokesperson likewise said that it would report on the internal inquiry in the second half of April.

VW Emissions Scandal

Back in September, Volkswagen admittedly employed a sophisticated software to manipulate emissions tests on nearly 500,000 diesel cars distributed in the United States.

Winterkorn, who served as CEO of the carmaker since 2007, stepped down in the same month following the controversy.

The VW board then appointed Porsche brand chief Matthias Mueller as being the company's new boss.

VW's US CEO Said He Didn't Know About It

When Michael Horn, the company's U.S. chief executive, appeared in front of the House of Representatives' Energy and Commerce Committees Oversight and Investigations subcommittee, he pointed his fingers to some software engineers of VW for the emissions cheating scandal. He also added that he was unaware of the defeat device equipped into some of the company's automobiles.