In January 1940, the Germans took over a mental hospital in Koscian, about fifty kilometres 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 led into a truck with airtight cargo space. Next step was to close the cargo space doors and start the engine. The engine exhaust was then through a hose led into the cargo space and all inside were killed by carbon monoxide. The truck were then driven to a wooded area called Jarogniewice, about ten kilometres north of Koscian. There prisoners from Fort VII prison in Poznan had pre-dug mass graves where the newly killed 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 the bodies were ravaged by 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).