In April 1943 Mackiewicz was invited by the international Katyn Commission, headed by German occupying authorities, to the site of Katyn massacre. Upon consent of the Polish government-in-exile, he assisted in the first excavations of the mass graves of the Polish officers killed by Soviet NKVD there in 1940. Upon his return to Vilnus, the local German-sponsored daily "Goniec Codzienny" published an interview with Mackiewicz titled "Widziałem na własne oczy" ("I saw with my own eyes"). He later arrived in Italy where he worked for the II Corps (Poland) and, in this capacity, he edited a compilation of documents related directly to the Katyń Massacre under the title "Zbrodnia katyńska w świetle dokumentów" (The Katyn murder in light of new evidence), published in 1948 with an introduction by General Władysław Anders. At the same time he wrote his own book under the title "Katyń. Zbrodnia bez sądu i kary" (Katyn. Murder without Trial nor Sentence). Its first Polish language publication was destroyed by the publisher in London, UK, for political reasons. In 1949 he published its German language version, "Katyn – ungesühntes Verbrechen", in Zurich, Switzerland. In 1951 he published the English-version of his book under a title "The Katyn Wood Murders", the very first book in English on the subject (Italian version in 1954 as "Il Massacro della foresta di Katyn", in Spanish in 1957 as "Las Fosas de Katyn", in Russian published in Canada in 1988 as "Катынь"). In 1952 he testified before US Senate Committee known as "Select Committee to Investigate and Study the Facts, Evidence, and Circumstances of the Katyn Forest Massacre" about the genocidal nature the Katyń Massacre (the Polish version of "Katyń. Zbrodnia bez sądu i kary" was published in 1997 in Poland, and again under different title: "Sprawa mordu katyńskiego. Ta książka była pierwsza" in 2009).