With the closing of the Inner German border officially in 1952,[7] the border in Berlin remained considerably more accessible than the rest of the border because it was administered by all four occupying powers.[2] Accordingly, Berlin became the main route by which East Germans left for the West.[8] The Berlin sector border was essentially a "loophole" through which Eastern Bloc citizens could still escape.[7] The 4.5[9] million East Germans that had left by 1961 totaled approximately 20% of the entire East German population.[10] The loss was disproportionately heavy among professionals—engineers, technicians, physicians, teachers, lawyers and skilled workers.[10] The "brain drain" of professionals had become so damaging to the political credibility and economic viability of East Germany that closing this loophole and securing the Soviet-imposed East-West-Berlin frontier was imperative.[11]