Having retaken the Katyn area almost immediately after the Red Army had recaptured Smolensk, around September–October 1943, NKVD forces began a cover-up operation. They destroyed a cemetery the Germans had permitted the Polish Red Cross to build and removed other evidence. Witnesses were "interviewed" and threatened with arrest for collaborating with the Nazis if their testimonies disagreed with the official line. As none of the documents found on the dead had dates later than April 1940, the Soviet secret police planted false evidence to place the apparent time of the massacre in mid-1941, when the German military had controlled the area. NKVD operatives Vsevolod Merkulov and Sergei Kruglov issued a preliminary report, dated 10–11 January 1944, that concluded the Polish officers were shot by German soldiers.