Although the final report is scheduled for publication at the end of April 2016, German business daily Handelsblatt has learnt about the investigation's conclusions from anonymous sources within the KBA.
The KBA began its investigation back in September 2015, not long after Volkswagen officially confirmed to US authorities and the public that it had used to emissions testing defeat devices to produce compliant results during laboratory testing in around half-a-million US market Audi and Volkswagen cars.
A total of around 11 million vehicles worldwide equipped with the EA189 turbo-diesel engine are affected by the so-called dieselgate scandal.
As part of the KBA's investigation, the authority examined the real world and laboratory emissions of 60 diesel vehicles, from a variety of manufacturers, sold in Germany.
According to the newspaper's sources, the KBA found "strange irregularities" in the emissions from some diesel cars from other marques. Despite that, though, these cars were still operating within legal limits.
After months of testing, the KBA's investigators have concluded that only vehicles from the Volkswagen Group used defeat devices to produce fraudulent results.
Raw results from the investigation have been reportedly been to sent to all the automakers who had their cars tested. Full results and the investigation's final recommendations will be published at the end of the month.