 Complete the following phrase:  In April 1943, in the Katyn Forest near Smolensk in the Soviet Union, occupying German troops discovered eight large graves containing the remains of thousands of the Polish Army officers and intellectual leaders who had been interned at the prisoner-of-war camp at Kozielsk. Bodies of the prisoners who had been housed at Ostashkov and Starobielsk were discovered near Piatykhatky and Mednoye, respectively. Collectively, these murders are known as the 