From Eavesdropping on Hell:
Gruesome details from coded Nazi messages that Britain intercepted beginning in 1941 could have confirmed and exposed the scope of German genocide well before 1945, when Allied troops liberated the death camps. Eavesdropping on Hell, written by Robert J. Hanyok, a historian with the National Security Agency’s Center for Cryptologic History, which was quietly released last month, reports that intelligence gleaned throughout the war from German military and police communication and from foreign diplomats provided lurid, though often fragmentary and episodic, accounts of massacres, deportations, and even statistics on the killing in concentration camps. One message, declassified in 2000, was intercepted on Jan. 11, 1943. It specified the number of Jews killed under “Operation Reinhard” at four death camps – Lublin, Belzec, Sobibor, and Treblinka – through 1942: 1,274,166.