“Initially, most prisoners were members of the Communist Party of Germany, but as time went on different groups were arrested, including “habitual criminals”, “asocials”, and Jews.”
https://en.wikipedia.org/wiki/Nazi_concentration_camps
And Stalin fingered the Communists for Hitler, because Stalin was afraid that if the Communists took over in Germany, they would try to replace the Soviets as the leaders in the Communist movement. Ernst Thaelmann was more useful as a “martyr” to the Soviets.