This chapter examines how the Combined Services Detailed Interrogation Centre (CSDIC) in Britain developed its sophisticated system of interrogating German soldiers. While CSDIC’s use of microphones is now widely known, the process that produced its intelligence remain poorly understood. The chapter shows that CSDIC was initially designed to gather pieces of information overheard among soldiers, but eavesdropping alone soon proved insufficient for the scale and complexity of the war. Soon enough, CSDIC UK had become the final step in a much larger screening process that sifted hundreds of thousands of soldiers to identify a select group for the bugged sites. The actual process in its camps then formed a cycle: statements obtained in direct interrogation were checked against private conversations between prisoners, which in turn revealed new leads for direct interrogation. Microphones, therefore, were part of a broader interrogation method rather than its entirety. The chapter also highlights the people at the centre of this system, whose work yielded a remarkable body of intelligence, yet its emotional and psychological toll remained largely concealed by secrecy well after the war.

CSDIC is best known for the transcripts of prisoner conversations it generated through covert listening. Yet these dialogues stand largely on their own: they contain no material to explain why particular prisoners and conversations were monitored. The chapter argues that to understand the purpose of these transcripts, we need to look beyond them, to the other kinds of reports CSDIC produced that were not derived from microphones. Numbering roughly 2,500 documents, these reports chart a transformation in the information Britain sought to extract from these prisoners. The chapter shows how CSDIC was initially concerned with military information, but that it soon expanded in both reach and ambition, moving from tactical and operational intelligence to more specialised studies aimed at understanding enemy morale and shaping post-war occupation planning. Alongside these efforts, its microphones were also capturing references to atrocities and a growing awareness among some soldiers of the genocidal intent behind them. Yet such revelations remained subordinate to institutional priorities about protecting CSDIC’s methods even after 1945.