In January 1940, the Germans took over a mental hospital in Koscian, about five miles south of Poznan. In mid-January, about 250 patients were murdered by carbon monoxide poisoning. The patients had first been given a soothing syringe and then loaded into a truck with an airtight space. Then the truck was started and the exhaust was led into the space that the patients were sitting in. When all the patients were dead, the truck drove to a wooded area called Jarogniewice, about a mile north of Koscian. Prisoners from Fort VII prison in Poznan had dug mass graves in which the patients were buried. A week later, another 250 patients were murdered in the same way. In early February 1940, about 2,750 Jewish and non-Jewish patients were taken from mental hospitals in Germany to Koscian where they were murdered. In less than a month, about 3,300 patients had been murdered in Koscian and buried in Jarogniewice. In February 1944, the germans dug up the mass graves and destroyed the bodies with the help of slaked lime (a substance that, upon contact with water, becomes corrosive).

Current status: Monument (2015).

Location: 52°09'56.26"N 16°41'34.78"E

Get there: Car.

My comment:

There are two monuments located along a forest road. There are no signs so you need to know where to look.

Follow up in books: Friedlander, Henry: The Origins of Nazi Genocide – From euthanasia to the final solution (1995).