The sector borders inside the city could in general be used freely for passage out of the German Democratic Republic, even after the border between the Federal Republic of Germany and the GDR had been continually closed off, starting in 1952. The outer border of West Berlin, which was also the border between West-Berlin and the GDR, had also been closed down in 1952. During the night of 12 to 13 August 1961 the National People's Army, the German Border Police, the Volkspolizei and the Combat Groups of the Working Class locked down all passages between the Soviet sector and the three West sectors; construction of border protection facilities began.