The VW Autostadt in Wolfsburg, Germany | Danie Reinhardt/EPA German car companies ran cartel going back to the 90s: report Case could be one of the biggest in German industrial history, news magazine Der Spiegel says.

Major German car companies agreed in secret working groups going back to the 1990s on technical issues, suppliers and costs in what may be one of the biggest cartel cases in the country's history, news magazine Der Spiegel reported Friday.

Volkswagen flagged the practice in a briefing it sent to German competition authorities, according to the magazine. Volkswagen reportedly said it struck agreements with Audi, Porsche, BMW and Daimler in secret working groups on a number of technical and economic aspects of their cars and production processes. Daimler also submitted a report to the competition authorities, the magazine said.

The working groups even set the stage for the Dieselgate scandal, Der Spiegel reported. The companies agreed on the size of tanks containing AdBlue fluid, which reduces exhaust emissions — but the tanks proved too small to do the job so manipulation became the only way to keep emissions readings low, the magazine said.

The companies declined to comment on details of the investigation, Der Spiegel said.