Starting in the spring of 1939, the Nazi regime systematically killed at least 200,000 mentally and physically disabled people that it deemed as “life unworthy of life” at secretive euthanasia camps spread across the Reich.

In a precursor to the horrors carried out at their concentration camps during the Holocaust, Nazi authorities would lure patients taken from mental hospitals into a shower room at the euthanasia centers and kill them by carbon monoxide poisoning. The patients’ bodies would then be either cremated or dissected to be studied by the Reich’s scientists, while their families were given bogus death certificates listing a fictitious cause and date of death.

Over 75 years later, researchers at Germany’s Max Planck Society are scouring archives and tissue sample collections in an effort to find out more about who these people executed in Nazi gas chambers were and to take some moral responsibility for the unethical research conducted by the organization’s predecessor, the Kaiser Wilhelm Society.

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“We want to find out who the victims were, uncover their biographies and their fates, and as such give them part of their human dignity back and find an appropriate way of remembrance,” Heinz Wässle, an emeritus director of the neuroanatomy department at the Max Planck Institute for Brain Research in Frankfurt, Germany, told Science Magazine.

Despite condemnation by the Catholic Church during the early 1940s and ghastly revelations laid out by doctors during the Nuremberg trials following World War II, the full extent of the Nazi extermination program is still not completely known.

What is known is that the program began in the spring of 1939 with the killing of disabled children but soon broadened to include adult disabled patients living in institutional settings. Adolf Hitler signed a secret authorization as war broke out across Europe in order to protect those participating in the killing and Führer Chancellery director Phillip Bouhler and Hitler’s physician Karl Brandt ran the so-called “T-4” program.

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T-4 administrators distributed questionnaires to all public health officials, public and private hospitals, mental institutions, and nursing homes under the guise that they were gathering statistical data and seeing who could work as laborers in the war effort. Unbeknownst to many staff members at these hospitals, those patients deemed incapable of work were taken to one of six euthanasia camps and systematically killed in the gas chambers.

“The directors of many of these mental hospitals would exaggerate the incapacity for work on their patients because they wanted to spare them from labor groups,” Michael Bryant, a professor of history and legal studies at Bryant University, told FoxNews.com. “But in fact they were consigning these patients to death.”

Interest in the brain tissue samples of Nazi victims first came up in the 1980s when a German journalist linked slides collected by Julius Hallervorden - the director of the Kaiser Wilhelm Society’s neuropathology department during World War II who accepted hundreds of brains of euthanasia victims - to a group of 38 children killed during one day in October 1940. This led the Max Plank Society to destroy about 100,000 slides dating to the Nazi era and in 1990 another neuropathology center, the Max Plank Institute for Psychiatry, ceremonially buried their own slides from World War II.

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In 2015, however, researchers came across a cardboard box containing 100 brain sections and further investigation at the institute turned up even more slides.

“The investigations into the brain sections belonging to the estate of the doctor and brain researcher Julius Hallervorden, rediscovered in the spring of 2015, have prompted the President of the Max Planck Society to launch a total review of all those Max Planck Institutes that still own collections of human specimens,” the society said in a press release. “Initial investigations at the Max Planck Institute of Psychiatry had shown that the Institute still possesses brain sections which actually should have been interred in Munich's Waldfriedhof in 1990.”

A three-year investigation is now underway to uncover any remaining specimens and help track down who these people were as well as gain a better understanding of the horrific science procedures carried out in Nazi Germany and their role in atrocities like the Holocaust.

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“It’s rightfully so that we focus on the horrors perpetrated against the Jews, the Poles and the Soviets by the Nazis during the war,” Bryant said. “But we frequently lose sight of the first victims of the Nazis' systematic murders.”