Volkswagen Group engineers admitted they helped fabricate carbon-dioxide emissions data, the German newspaper Bild am Sonntag reported Sunday. The engineers said former VW CEO Martin Winterkorn placed unreasonable demands on them, which drove the decision to cheat.

"Employees have indicated in an internal investigation that there were irregularities in ascertaining fuel-consumption data," Reuters quoted a Volkswagen representative as saying. "How this happened is subject to ongoing proceedings."

The Volkswagen scandal exploded in September after scientists at a University of West Virginia laboratory tested several of the company's diesel vehicles and found their carbon emissions were as much as 40 times the legal limit in the U.S. VW was able to cheat on emissions tests by employing a type of software, a so-called defeat device, that indicated the vehicles' emissions were far lower than they were in reality.

The software affected at least 11 million Volkswagen diesel vehicles worldwide, including about 500,000 in the U.S. The parent company also owns the Audi and Porsche brands, and many vehicles produced by those units also were equipped with defeat devices. Several of the firm's top executives, including Winterkorn, have resigned, as an investigation continues into who knew about the software.

At a 2012 auto show, Winterkorn said he wanted Volkswagen to cut its carbon emissions by 30 percent by 2015. Company engineers, who apparently would not dare to tell the CEO that this goal was unrealistic, turned to cheating to compensate for missing the emissions goal, according to the article in Bild.

Volkswagen engineers admitted their part in the cheating scheme in October, and the internal whistleblowers reportedly spurred the company’s efforts to come clean about dirty diesel. Countries such as the U.S., Germany and France have launched separate investigations in addition to VW’s internal probe to discover who was responsible for the cheating.