obviously they're not going to send telegraphs saying "ok you need to kill these jews". they knew this isn't something anyone outside of the ss could ever know there were in fact several intercepted communications discussing jewish extermination details, but they didn't contain very clear language since again they obviously are not going to openly write about killing civilians the most significant enigma-broken message is one reporting the total number of of executed jews at the operation reinhard extermination camps lublin, belzec, sobibor, and treblinka at the time the message was sent: 1.2 million. the document is marked "state secret" and was sent to eichmann, one of the main bureaucrats in charge of organizing the extermination of jews in europe. in terms of other written documentation: there's the letter from himmler to the nazi governor of poland when he asked him what should be done about all the jews. himmler replied "als partisanen auszurotten" (treat all jews as partisans and exterminate them all). before this message was sent jews were already being shot en masse in poland by execution squads devoted to nothing but killing civilians in general, though, they knew they needed to keep this outside of writing. hence why it was verbal orders or in the case, himmler giving speeches to ss leadership:   _speeches > i am now referring to the evacuation of the jews, the extermination of the jewish people. it's one of those things that is easily said: 'the jewish people are being exterminated', says every party member, 'this is very obvious, it's in our program, elimination of the jews, extermination, we're doing it, hah, a small matter.' [...] to have endured this and at the same time to have remained a decent person  with exceptions due to human weaknesses  has made us tough, and is a glorious chapter that has not and will not be spoken of.
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