There was no such organization as an "Allied Investigating Commission" in such a general form. The United States, Great Britain and other Allied nations had already formed the "War-Crimes Commission of the United Nations" during the Second World War. This met in London for the first time in October, 1943 to organize the collection of material about war-crimes. The Commission formed the starting point for the International Military Tribunal at Nuremberg.
The trial against those responsible for the Mauthausen concentration camp was processed by a United States court in Dachau. Here the question of poison-gas murders was investigated. It would have been absurd for the very authority that carried out these extensive trials to produce such a "document". Besides, the question of poison gassing in the Mauthausen concentration camp had already been dealt with during the Allies' Nuremberg trials.
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