Research into the wartime civil registries of one of the civil registry offices in Upper Silesias (Poland) discovered many records that corresponded to deaths of inmates from the "Reichsautobahnlager Annaburg" and "Zwangsarbeitslager Niederkirch" camps.
A certificate of death for Johan Slager as discovered there, stated that he died on 23 December 1942 in the forced labourcamp Niederkirch. In it was no mention of an official cause of death.
A deportation list in the Central Database of Yad Vashem shows that Johan Slager has been deported on 1 September 1942 with Convoy VII from Mechelen in Belgium in the direction of Auschwitz.
Private collection of Edward Haduch, Kedzierzyn-Kozle (Poland), the death certificate of Johan Slager from the Civil Registry (Standesamt) of Gross Walden and the Cental Database of Yad Vashem.
However, by establishing the date of death of Johan Slager, the official date is maintained as stated after the war by the Dutch Department of Justice.