The newspaper mentions an ornament with the inscription "Invisible". If we assume that the ornament is somehow related to invisibility, then the first and only association is the One Ring (which Fyodor Sumkin lugged all the way to Mount Doom). When wearing that little ring, it made Fedya invisible. In the Witch's signature, there is a Balrog, a demon from the same "Lord of the Rings". Logic clear?)
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And it really wouldn't hurt to listen to the others...
Besides, I suspect the Witch of being a maniac with a circular saw, and Amko of being a
And also, I don't see anyone else left who could be pinned with the evidence for the mafia...