In June 1945, during investigations by US Forces into the former gassing facility at Hartheim, the American investigating officer Charles Dameron broke open a steel safe in which the Hartheim statistics were found. This was a 39-page brochure produced for the internal purposes of the Nazi euthanasia programme (Aktion T4), and contained monthly statistics of the gassing of mentally and physically handicapped patients (called "disinfection" in the document) carried out in the six euthanasia institutions on the territory of the Reich. In 1968 and 1970 an ex-employee of the establishment revealed, as a witness, that he had to compile the material at the end of 1942. The Hartheim statistics included a page on which it was calculated that "disinfecting 70,273 people with a life expectation of 10 years" had saved food in the value of 141,775,573.80 Reichsmarks.
According to the Hartheim statistics, a total of 18,269 people were killed in the gas chamber at the Hartheim euthanasia centre in the period of 16 months between May 1940 and 1 September 1941, as follows:
These statistics only cover the first extermination phase of the Nazi's euthanasia programme, Action T4, which was brought to an end by Hitler's order dated 24 August 1941 after protests by the Roman Catholic Church.
In all it is estimated that a total of 30,000 people were executed at Hartheim. Among those killed were sick and disabled persons as well as prisoners from concentration camps. The killings were carried out by carbon monoxide poisoning.
Just three days after the formal end of Action T4, a lorry arrived at Hartheim with 70 Jewish inmates from Mauthausen concentration camp who were subsequently executed there. The Hartheim killing centre achieved a special notoriety, not just because it was where the largest number of patients were gassed, but because as part of Action 14f13 Hartheim was also the institution in which the most concentration camp prisoners were executed. Their numbers are estimated at 12,000.