Complex sentence: The interrogation of German prisoners indicated that uranium and thorium were being processed in Germany, mostly at the "Auergesellschaft" plant at Oranienburg, so Groves arranged for the plant to be bombed on 15 March 1945.
Split into simpler sentences: The interrogation of German prisoners indicated that uranium and thorium were being processed in Germany. It was discovered that most was sent to the "Auergesellschaft" plant at Oranienburg, so Groves arranged for it to be bombed on 15 March 1945.