The first evidence picked up by allied intelligence about the development of the death camps was in late 1941 and 1942 through decrypts of Enigma messages sent by the police and by the SS. This suggested that the Nazi rule of terror in the occupied East featured regular atrocities. The numbers of deaths being reported in these messages shocked the intelligence officers but they failed to realize that this added up to a full-scale extermination policy. Rumors of the horrors of the camps filtered through Jewish groups and the Polish underground, back to London and Washington. A telegram from Gerhart Riegner of the World Jewish Congress arrived in London. Based on reports smuggled out of Germany it was the first evidence of a co-coordinated Nazi plan to annihilate the entire Jewish population of the Occupied territories. But still, officials were skeptical of the idea that vast death camps had been constructed somewhere in the East. Chaim Weizmann, President of the Jewish Agency, made a formal request to the British and American governments to bomb the camps or the railways leading up to them, to prevent further killing from taking place.
It was quickly decided that the use of heavy four-engined bombers would be counter-productive. With the accuracy of bombing techniques at this point of the war, a bombing raid led by, say, American B-17 Flying Fortresses would run the risk of destroying the entire camp and killing tens of thousands of inmates. No Allied commander wanted to take this risk.