The Bubeneč Popper house and surrounding land was confiscated by the Nazi German authorities on 16 March 1939, a day after the German occupation of Czechoslovakia, and was subsequently used as the Prague headquarters of the Gestapo (German secret state police). It was in this period of time that the embassy was fitted with a series of tunnels which housed Gestapo archives as well as a security hallway connecting the tunnels to the embassy. In 1945, after the end of the war, the family attempted to reclaim the property, but were prevented from doing so due to the nationalisation decrees of Beneš, who returned to Czechoslovakia as President. Although Beneš was aware that the Czech–Jewish banker was still alive and intended to return to Prague, the property was granted to the Soviet Union by Beneš as a gift in thanks for the Soviet liberation of Czechoslovakia from Nazi rule. The building was used by the Soviets to accommodate the embassy of the Soviet Union to Czechoslovakia. Following this shift in ownership, the Soviets worked to integrate the tunnels and secure rooms which were added by the Nazi government. It is a widely held belief that many of these rooms were used by KGB Line X officers for means of espionage and counter intelligence.