This article examines and contextualizes a riot that occurred in Dar es Salaam in 1959, in the peri-urban neighbourhood of Buguruni. The riot involved accusations that security guards and police were abducting neighbourhood residents and killing them in order to use their blood for the preparation of magical medicines. Those who abducted Africans for this purpose were popularly termed mumiani. Their rumoured existence is examined in the wider context of Dar es Salaam's rapid urbanization, its peri-urban politics and land conflicts, and its systems of law and knowledge. The article also explores the many possible interpretations of this riot. Drawing on interviews with local residents, court testimonies, official correspondence, newspaper accounts, and colonial memoirs, the article constructs a historical account of the riot's location, Buguruni, as well as a narrative of the riot itself and the subsequent legal actions. Such a violent event raises questions about the relationship between historical evidence and causality, as well as questions about contextualizing major events that fit awkwardly into prevailing historical narratives.