Stangl studied the camp operations and management of Bełżec, which had commenced extermination activity. He then accelerated the completion of Sobibor. Around that time Stangl also had further dealings with Wirth, who was running extermination camps at Bełżec and Chelmno. Between 16 and 18 May 1942, Sobibor became fully operational. However, Stangl quickly realized that the extermination process was being encumbered by constant turnover among its prisoner labor force. He ended arbitrary culling of "work Jews" and established semi-permanent work teams, each overseen by a kapo. In the three months before Stangl was transferred to Treblinka, Yitzak Arad estimates that approximately 90,000 Jews were killed at Sobibor.