He was sentenced to death for the killing of Canadian prisoners of war and was executed in 1949.

Siebken, a driving and riding instructor, joined the SS and the NSDAP in 1931 and was one of the original members of the SS-Stabswache (March 1933) and its successor the Leibstandarte SS Adolf Hitler (LSSAH).

He was found guilty in the shootings of Canadian prisoners of war from the Queen's Own Rifles during the Battle of Le Mesnil-Patry and hanged on 20 January 1949.

[3] Following the reburial of executed war criminals in Hamelin in 1954, the cemetery became the focal point for veterans' reunions, with distinct Nazi overtones.

In 1959, for example, the convention of the lobby group and revisionist organisation of former Waffen-SS members, HIAG, concluded with "comrades gathering around [Siebken's] tomb" and laying a wreath.