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Rapid expansion (1937–1939)

By the end of June 1938, the prisoner population had expanded threefold in the previous six months, to 24,000 prisoners. The increase was fueled by arrests of those considered habitual criminals or asocials. According to SS chief Heinrich Himmler, the "criminal" prisoners at concentration camps needed to be isolated from society because they had committed offenses of a sexual or violent nature. In fact, most of the criminal prisoners were working-class men who had resorted to petty theft to support their families. Nazi raids of perceived asocials, including the arrest of 10,000 people in June 1938, targeted homeless people and the mentally ill, as well as the unemployed. Although the Nazis had previously targeted social outsiders, the influx of new prisoners meant that political prisoners became a minority.

To house the new prisoners, three new camps were established: Flossenbürg (May 1938) near the Czechoslovak border, Mauthausen (August 1938) in territory annexed from Austria, and Ravensbrück (May 1939) the first purpose-built camp for female prisoners. The mass arrests were partly motivated by economic factors. Recovery from the Great Depression lowered the unemployment rate, so " work-shy " elements would be arrested to keep others working harder. At the same time, Himmler was also focusing on exploiting prisoners' labor within the camp system. Hitler's architect, Albert Speer, had grand plans for creating monumental Nazi architecture. The SS company German Earth and Stone Works (DEST) was set up with funds from Speer's agency for exploiting prisoner labour to extract building materials. Flossenbürg and Mauthausen had been built adjacent to quarries, and DEST also set up brickworks at Buchenwald and Sachsenhausen.

Political prisoners were also arrested in larger numbers, including Jehovah's Witnesses and German émigrés who returned home. Czech and Austrian anti-Nazis were arrested after the annexation of their countries in 1938 and 1939. Jews were also increasingly targeted, with 2,000 Viennese Jews arrested after the Nazi annexation. After the Kristallnacht pogrom in November 1938, 26,000 Jewish men were deported to concentration camps following mass arrests, overwhelming the capacity of the system. These prisoners were subject to unprecedented abuse leading to hundreds of deaths – more people died at Dachau in the four months after Kristallnacht than in the previous five years. Most of the Jewish prisoners were soon released, often after promising to emigrate.