Three major German auto companies are facing mounting criticism and scrutiny following a report last week alleging that they had funded research involving diesel exhaust inhalation tests performed on animals.

Volkswagen (ETR: VOW3), BMW (ETR: BMW), and Daimler (ETR: DAI) have come under fire for the 2014 experiment, conducted on monkeys by the Lovelace Respiratory Research Institute in Albuquerque, New Mexico. Animal testing raises serious ethical questions about the companies' practices. Earlier this week, German newspapers Süddeutsche and Stuttgarter Zeitung reported that the same manufacturers had paid for similar exhaust inhalation tests conducted on human subjects. The European Research Association for Environment and Health in the Transport Sector (EUGT), a now-disbanded car lobbying group that included Volkswagen, BMW, and Daimler, had conducted these tests on "25 young subjects." In response to these revelations, all three companies have condemned the experiments and announced the firings or suspensions of employees with knowledge of the testings.

The 2014 Albuquerque tests were performed on ten monkeys kept in chambers pumped with diluted exhaust from a diesel Volkswagen Beetle. News of the tests has sparked outrage from animal rights groups and other observers. German chancellor Angela Merkel pushed the companies to reveal the full extent of their scientific research.

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The outcry against German automakers comes only a few years after a high-profile scandal involving Volkswagen and the same diesel technology. In late 2015, Volkswagen was exposed for illegally using software to hide the true measurements of emissions by its diesel automobiles. The case led to international condemnation of the company and resulted in admissions of guilt by its top executives, as well as a prison sentence for one U.S.-based Volkswagen executive, Oliver Schmidt.

With the latest revelations about animal and human testing, the companies themselves have reacted with public censure of the practices. Both BMW and Daimler have put out statements distancing themselves from the experiment and from Volkswagen. Daimler announced on Wednesday that it was suspending an employee in response to the news. BMW has also announced the suspension of an employee who served as the company's representative on the EUGT.

Volkswagen, which has borne the brunt of the outrage, has moved swiftly to control the damage. In a statement released January 30, Volkswagen announced that its head of government relations and sustainability, Thomas Steg, was voluntarily stepping down. The suspension of Steg, who admitted that he had known about the experiments at the time they were conducted, was the first high-profile casualty of this latest scandal. According to reports by Spiegel Online, Volkswagen's chief executive, Matthias Müller, on Monday condemned the experiments as "unethical and repulsive," and promised that there would be internal consequences for the misconduct.