The investigation into emissions fraud at Volkswagen reportedly widened over the weekend after the automaker acknowledged US and German regulators were examining why some Audi cars and SUVS behaved differently during tests than they did on the road.

The disclosure raises suspicions that Volkswagen used a new type of emissions cheating software in some Audis, the New York Times (NYT) said.

In a statement cited by the NYT, Volkswagen confirmed a report in Germany's Süddeutsche Zeitung newspaper some Audis with automatic transmissions behaved differently when the cars were being tested. If the software that controlled the transmission detected testing conditions, according to the newspapers, the cars shifted in a way that would produce less carbon dioxide. In normal use, the cars used a shifting sequence intended to provide better performance.

Audi did not identify the models involved but, according to a lawsuit filed on behalf of Audi owners in United States District Court in Minnesota on Thursday, they include 100,000 A6 and A8 sedans and Q5 and Q7 SUVs with three-litre petrol engines made up to the end of May 2016, the NYT said. The cars sensed that they were undergoing stationary tests on rollers if the steering wheel did not move more than 15 degrees.

According to the report, representatives for Volkswagen and Audi acknowledged regulators were looking at why some models shifted differently during tests. But the company suggested that any discrepancies were not a deliberate attempt to cheat.

"In the testing situation," Audi said in a statement, "dynamic shift programmes can lead to incorrect readings and results that cannot be reproduced."

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Both the New York Times and Bild am Sonntag cited an internal VW document which recorded the outcome of test drives of new vehicles in southern Africa in February 2013. According to the document, a high-ranking manager asked, "When will the cycle-optimised shifting program be available?"

'Cycle optimisation" is code for software or other equipment that allows cars to perform differently when they are undergoing official test cycles on rollers in a lab. According to the reports, the document noted "the shifting program should be configured to be 100% active on rollers but only 0.01% for customers."