A number of popular German brands — including Hugo Boss, Daimler-Benz, Porsche, and BMW — had connections to or business arrangements with the Nazi party and/or the German military under Hitler. But the brand that may be most commonly linked with the Third Reich is Volkswagen, a company that had, for the last 18 years, contracted a noted historian to research VW’s embarrassing origins, including its use of forced labor. However, at a time when an ongoing emissions scandal has called the carmaker’s commitment to transparency into question, VW and the academic have gone their separate ways.

The NY Times reports on the split between VW and researcher Manfred Grieger, who has previously published work — some of it funded by VW — chronicling the use of slave labor by Volkswagen and Audi (which is has been owned by VW since the 1960s), noting that the carmaker chose to not renew its contract with Grieger when it recently expired.

Additionally, during his nearly two-decade stint working with VW, Grieger allowed other researchers and reporters to access the company’s historical archive. This was viewed as a refreshingly transparent move by a company whose early years are so intertwined with a shameful era of German history.

While Grieger isn’t commenting on the reason behind the split, 75 of his colleagues in German academia are, penning an open letter to Volkswagen and arguing that the historian’s contract wasn’t renewed because Grieger was critical of a 2014 report about Audi’s labor practices during the Third Reich. He claimed the 500+ page study downplayed the car company’s use of forced labor and the company’s connections to the Nazi party.

“Just this brief discussion in an academic journal then led to talk that Grieger be put on a short leash and limited in his academic freedom, which in turn led the prominent historian to leave,” reads the open letter.

There are also indications that Grieger’s work might have rubbed some of the company’s major stockholders the wrong way, as it directly implicated their famous ancestors.

In the 1930s, Hitler tasked Ferdinand Porsche, founder of the world-famous sports car brand, with designing what would eventually become known as the VW Beetle. Additionally, Porsche — along with his son-in-law Anton Piëch — supervised the development of the Reich’s prestige manufacturing facility in Wolfsburg, the city that VW still calls home.

Not only did Grieger’s work claim that forced labor was used at the Wolfsburg factory, but his research also showed the Piëch led a small post-war militia intent on continuing to fight the Allied forces.

The connection between these families and Volkswagen continued long after the war, with Piëtch’s son Ferdinand becoming CEO of Volkswagen in 1993. Even after retiring, he remained as Chairman of the Board at VW until April 2015, only a few months before the “dirty diesel” scandal rocked the company. The Times notes that the Porsche and Piëch families now hold the majority of voting shares in Volkswagen.

In a statement, Volkswagen provides no explanation for parting ways with Grieger, other than to say his contract has expired.

“The fact is that Volkswagen continues to recognize the achievements of Dr. Grieger and to thank him for the work performed,” reads the statement to the Times. “Furthermore, the fact is that Volkswagen has examined its history as an enterprise consistently, honestly and strongly, and will continue to do so.”

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