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There is an interrogation report and Kolgrim's letter which suggest that he was killed while trying to cleanse the crypt of wraiths. However Geralt can overhear another version, told by peasant women not far from the White Orchard inn:
— Me gran tol' me about the smith's boy. Lad went missin' one day, so folk nabbed a witchman that was huntin' nearby. Started pokin' 'im, askin'. Well, he started twistin' and squirmin' so wham-bam - they knocked his head off!
— Aye, so it were. 'Cept it was some drowner that had supped on the kid. Witcher had naught to do with it!
— What did the folk do?
— Tain't like they coulda sewed his head back on! Word is they laid his corpse to rest - hush-hush in a crypt.
It suggests that Kolgrim was actually killed by angry peasants who made it look like wraiths did it. It's also clear from the story that it took place quite a long time in the past (which is also apparent from the condition of the ruined castle where interrogation took place).