The German government on Monday dismissed as unjustifiable any auto emissions testing on monkeys or people. "We condemn the experiments in the strongest terms".
"The supervisory bodies of those who issued these contracts have a special responsibility", he added.
According to the Stuttgarter Zeitung, the experiments were carried out at an institute of the University Clinic Aachen and involved the group having to breath in varying different concentrations of nitric oxide after which they were physically examined for any side-effects.
The statement, released at a time when only experiments with monkeys but not humans were publicly known, accused the researchers of having violated ethics rules and company values, even though an ethics commission had approved the study.
While two of the three auto makers have distanced themselves from the study, the results of which have never been published, Volkswagen has apologised for its part in the test.
The experiments are said to have been carried out back in 2014 in the United States by the "European Research Association for Environment and Health in the Transport Sector" (EUGT), which is founded by Daimler, VW, BMW and Bosch companies.
VW is already under heavy scrutiny over its role in the "dieselgate" scandal, in which the carmaker manipulated tests on about 11 million cars worldwide to make it appear they met air emissions tests, when in reality they exceeded them many times over when used on the road.
Daimler said, however, that all work commissioned with the EUGT was "accompanied and reviewed by a research advisory committee consisting of scientists from renowned universities and research institutes".
Daimler AG said it was "appalled by the nature and extent of the studies" and said that, though it didn't have any influence on the studies' design, "we have launched a comprehensive investigation into the matter". The company said that it knows that the experiments were wrong and contravened its own ethical standards.
The human study, carried out by Aachen University, involved studying the effects of exposing 25 subjects, mostly students, to low levels of nitrogen dioxide like those that could be found in the environment - from a 40-liter bottle, not a diesel engine. They have not yet responded to the news of the human experiments.
The test result stands in contrast to long-term medical studies drawing a link between nitrogen-dioxide and breathing problems, particularly among the young, the elderly and asthmatics.
It's alleged that EUGT forced monkeys to sit in sealed containers, watching cartoons to keep them placid while they inhaled fumes from a diesel Volkswagen Beetle.
Stephan Weil, who represents the German state of Lower Saxony, a Volkswagen shareholder, on the supervisory board, said the board was pressing the carmaker to urgently provide information about what the aim of the studies was. It has since emerged that the model used in the test was among those with cheat software to reduce emissions in test situations.