According to the British historian Antony Beevor, the order to kill the prisoners most likely came from the Spanish communist José Cazorla Maure or, more indirectly, from the Soviet advisor Mikhail Koltsov.

[7] Most prisoners, who were told they would be set free, were taken by trucks to fields outside Paracuellos del Jarama and Torrejón de Ardoz, where they were shot and buried in mass graves.

From the early days, news spread of the executions, which were denounced by foreign diplomats based in Madrid, including the consul of Norway, Felix Schlayer, who talked about the issue with Carrillo.

[8][9] On 8 December a plane carrying Dr. Georges Henny, an envoy sent by the International Red Cross, on his way back to France was shot down over Pastrana, northeast of Madrid.

The Republican authorities blamed the Nationalist air force for the attack, but on 21 December it was revealed that the plane had been shot down by Soviet-built airplanes with Soviet pilots.

Louis Delaprée, a French journalist who traveled in the same plane, who died weeks later because of his injuries, blamed General Aleksandr Mikhailovich Orlov, the Soviet NKVD's rezident in Spain, for the incident.