After the March 1944 German invasion of Hungary, the flow reversed and Slovak and Hungarian Jews fled back across the border to Slovakia. At this time, the approximately 800,000 Jews (as defined by race) in Hungary was the largest surviving population in Europe. The Jews in Hungary had been subjected to strict antisemitic legislation and tens of thousands had been murdered, both young men conscripted into labor battalions and foreign Jews deported to Kamianets-Podilskyi, but they had not yet been deported "en masse" or systematically exterminated. After the German invasion, the Working Group learned from sympathetic Slovak railway officials about the preparation of 120 trains for Jews deported from Hungary and relayed the information to Budapest, where it was received by the end of April.