Double Mirror

Three murders occurred in a row within three months. The victims were all middle-aged men. Their deaths were similar - they were hit on the back of the head with a blunt instrument, and the same strange fingerprint was left at the scene. But what is confusing is that this fingerprint belongs to one person, but not entirely to the same person - it shows a rare "mirror symmetry" feature, as if it is the left and right hands of the same person overlapping. What's even more bizarre is that a mirror was broken at each crime scene.