By late 1940, German armies had swept through most of Western Europe. The following year, Heydrich's SD was given responsibility for carrying out the Nacht und Nebel (Night-and-Fog) decree. According to the decree, "persons endangering German security" were to be arrested in a maximally discreet way: "under the cover of night and fog". People disappeared without a trace with none told of their whereabouts or fate. For each prisoner, the SD had to fill in a questionnaire that listed personal information, country of origin, and the details of their crimes against the Reich. This questionnaire was placed in an envelope inscribed with a seal reading "Nacht und Nebel" and submitted to the Reich Main Security Office (RSHA). In the WVHA "Central Inmate File", as in many camp files, these prisoners would be given a special "covert prisoner" code, as opposed to the code for POW, Felon, Jew, Gypsy, etc. The decree remained in effect after Heydrich's death. The exact number of people who vanished under it has never been positively established, but it is estimated to be 7,000.