We should not forget that during this period the Social Democrats had considerable powers in the area of law and order, because in 1928 one of their number, Carl Severing, was appointed Minister of the Interior of the Reich. The SPD took advantage of this to institute an extremely efficient reorganisation of the police, with the principal aim of setting up a special corps to prevent Bolshevik disturbances and uprisings.

Unfortunately they were not equally efficient and motivated in preventing and repressing [Fascist] gangsterism. The situation inevitably aggravated the historic fracture between Social Democrats and Communists that had already existed since the murders of Rosa Luxemburg and Karl Liebknecht — a fracture which experienced a particularly acute moment — a point of “no return” — in the events of Mayday 1929.

(Emphasis added. Source herein, and see here for more.)