As investigations into diesel-emission cheating deepen, accusations have arisen that the Volkswagen Group, Daimler and the BMW Group may have colluded on development of diesel engines and other areas.

Germany’s Der Spiegel reported on Friday that starting in the 1990s, the three automakers met in secret to discuss the technology, cost and suppliers of various components, including those for controlling diesel emissions.

One of the things discussed, for example, was said to be the size of DEF (Diesel Exhaust Fluid) tanks, used to reduce nitrogen oxides (NOx) in diesel vehicles. Large tanks would have been more expensive, so the automakers reportedly agreed on small tanks.

Der Spiegel cited a document submitted by Volkswagen in July 2016 and referred to another document by Daimler. Germany’s Federal Cartel Office has yet to comment on the report.

The report was published on the same day that the Volkswagen Group said it was voluntarily recalling 850,000 cars from the Audi, Porsche and Volkswagen brands fitted with V-6 and V-8 diesel engines in all major markets except the U.S. and Canada. That recall, which involves a software fix, will reduce NOx emissions in real driving conditions beyond the current legal requirements, the automaker said.

The V-6 engine being recalled is the same one discovered by regulators in the United States to be fitted with emission-cheating software. In the U.S., the VW Group has agreed to pay more than $20 billion in penalties and settlements and pleaded guilty to three felony charges due to the diesel scandal.

The latest recall comes at a time when the VW Group is under investigation over allegations of fraud and false advertising relating to the diesel scandal in its home market of Germany. Those investigations follow accusations by Germany’s official transport authority, the KBA, that some of the VW Group engines sold in Europe are fitted with emission-cheating software. The VW Group said in a statement it was working closely with the KBA in its investigation.

Meanwhile, over at Daimler, more than 3 million diesel vehicles, mostly from the Mercedes-Benz brand, were voluntarily recalled across Europe this week. The recall is an expansion of a recall from earlier this year for approximately 250,000 compact cars fitted with diesel engines.

Daimler’s recall also involves a software fix that the automaker says was developed using the latest knowledge it had gained during the development of next-generation diesel engines. While Daimler also faces investigations in Europe as well as here in the U.S., the automaker insists the recall was initiated to make its existing diesels cleaner—to alleviate any uncertainty owners may have over their cars' NOx emission levels. Like the VW Group, Daimler too is cooperating closely with German authorities.