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 Isidore Eduard Kaas as discovered there, stated that he died on 30 November 1942 in Camp Annaberg. In it was mentioned an official cause of death as “myocarditis”. (Herzmuskelentzündung).
The archives of the Jewish Council of Amsterdam list Isidore Eduard Kaas arriving 4 October 1942 in Camp Westerbork and that he had been deported "eastward" on 16 October 1942.
Private collection of Edward Haduch, Kedzierzyn-Kozle (Poland), the official death certificate of Isidore Eduard Kaas, issued by German authorities (Standesamt) of Annaberg and the Archive of the Jewish Council of Amsterdam.
However, by establishing the date of death of Isidore Eduard Kaas, the official date is maintained as stated after the war by the Dutch Department of Justice.