After the June 15, 1940 invasion and annexation of Lithuania by the Soviet Union, Mackiewicz worked temporarily as a labourer. In 1942, he witnessed the Ponary massacre of some 100,000 mostly Polish Jews by German SD, SS and the Lithuanian Nazi collaborators Ypatingasis būrys, which he described in his 1969 book "Nie trzeba głośno mówić" (One Is Not Supposed to Speak Aloud). At the end of 1942 (beginning of 1943) he was mistakenly sentenced to death by the resistance, for his work at "Gazeta Codzienna" and "Goniec Codzienny". The sentence was cancelled out (see below).