Relatives of victims of the murders in Hartheim started putting up the first memorial plaques in the castle’s arcade in the 1950s, on the external walls of the killing rooms. In 1969 the memorial room was established in the former reception room. By 1999, more than 40 memorial plaques had been put up here and in the former gas chamber.
As part of the new concept for the Memorial, the commemorative plaques were put back up in the arcade, on the external wall of the former crematorium. Any new plaques are now attached to the external wall of the castle (opposite the mausoleum).
While the euthanasia centre was in operation, the crematorium ashes, bone remains and non-valuable personal effects of the victims were stored, or later buried, in the former castle garden, to the east of the castle. Remnants of the killing installations were also disposed of here.
In 2001/2002 the area was the subject of an archaeological investigation. The human remains that were found were buried in September 2002, and the site was declared a cemetery. Within the cemetery area, the site of the burials is marked by the mausoleum, which Herbert Friedl designed. Other human remains were found during the work on the found objects, and these were buried in October 2009 in a second funeral ceremony.