Giebel was a Hamburg businessman who killed his aunt over a family dispute. His crime got tangled up in the investigation of the Middle-Aged Couple Murders—a series actually carried out by Johan Liebert—which the authorities had been pinning on Dr. Kenzo Tenma. The BKA brought in Inspector Heinrich Lunge to work the case.
Even as evidence piled up suggesting Tenma had nothing to do with Giebel's murder, Lunge didn't care about finding the real killer. He saw an opportunity. By convincing Tenma that his alternate personality Johan was responsible, he figured he could draw his longtime obsession to the aunt's house and finally catch him. So Lunge interrogated Giebel in an unsettling way, basically accusing him—less because he actually suspected him and more because he wanted to mess with him. Giebel kept insisting he was innocent, but Lunge wasn't interested in listening. He was using the guy as bait.
Desperate and convinced Lunge was determined to pin the crime on him, Giebel tailed the inspector to the aunt's place. When Lunge moved in to arrest Tenma, Giebel panicked and stabbed him, which obviously proved his guilt. Lunge just shoved him aside without much reaction and kept going after Tenma anyway. The wound was serious enough to make Lunge pass out from blood loss, though Tenma ended up saving him later. In the end, Lunge's superiors credited him with identifying and capturing Giebel, while Tenma managed to slip away.
Content compiled by AnimeList.moe from publicly available sources.
