The transcript of a telephone call between Nikita Khrushchev (the First Secretary of the Communist Party of the Soviet Union from 1953 to 1964) and Ulbricht, on 1 August in the same year, suggests that the initiative for the construction of the Wall came from Khrushchev.
On 9 August 1961, the NSA (the American National Security Agency) intercepted an advance warning information of the Socialist Unity Party's plan to close the intra-Berlin border between East and West Berlin completely for foot traffic.
On Saturday, 12 August 1961, the leaders of the GDR attended a garden party at a government guesthouse in Döllnsee, in a wooded area to the north of East Berlin. There, Ulbricht signed the order to close the border and erect a wall.
At midnight, the police and units of the East German army began to close the border and, by Sunday morning, 13 August, the border with West Berlin was closed. East German troops and workers had begun to tear up streets running alongside the border to make them impassable to most vehicles and to install barbed wire entanglements and fences along the 156 kilometres (97 mi) around the three western sectors, and the 43 kilometres (27 mi) that divided West and East Berlin. The date of 13 August became commonly referred to as Barbed Wire Sunday in Germany.