Zajančkauskas was born in Aukštadvaris. In 2005 and 2010, the U.S. federal courts asserted that Zajančkauskas deployed to Warsaw, Poland with a detachment of the Trawniki-trained guards who participated in the Nazi annihilation of the Warsaw Ghetto, an operation that triggered the Warsaw Ghetto Uprising and led to the extermination of more than 50,000 Jews. The federal courts rejected as incredible his claim that, as a captured member of the Lithuanian Army, he was only in charge of the canteen at the Trawniki training camp, and was never in Warsaw. Records released by Russian authorities in the 1990s were used by the OSI as evidence in the case. His name appears on a roster of 351 men deployed to the ghetto, a document which was captured by the Red Army in 1945.