Pluto Vol 6

From the VIZ Media edition

Pluto... Sahad... Goji... Abler... Mysterious figures somehow involved with the serial murders of the great robots of the world. Europol's top robot detective Gesicht has been put on the case, and he's mere steps away from discovering the horrifying truth behind the killer and his motives... Little does he realize that he's also steps away from discovering a horrifying truth of his own.

Source: Anime News Network Encyclopedia