Mathare is one of countless slums in Kenya that the government does not quite reach. There are no police stations here, or fire hydrants or roads. There are few toilets and the hillsides reek of fresh waste. Many of the dented metal kiosks advertise — in dripping hand-painted scrawl — paraffin for oil lamps, because despite the palatial homes in the neighborhood next door that light up like soccer stadiums at night, most Mathare dwellers have no electricity.

“These people live like beasts,” said Ms. Bokindo, one of the government officials in charge of Mathare.

The Mungiki did not start here. They came from the Kikuyu highlands north of Nairobi, that carpeted green, straight-off-a-postcard “Out of Africa” side of Kenya.

According to Hezekiah Ndura Waruinge, one of the Mungiki’s founders, the group began as a local defense squad during land clashes in the late 1980s between forces loyal to the government, which was dominated by the Kalenjin tribe, and farmers who were Kikuyu, a rival tribe.

The Mungiki, whose name means multitude in the Kikuyu language, modeled themselves after the Mau Mau, Kenya’s independence fighters who sprouted dreadlocks, took secret oaths and waged a hit-and-run guerrilla war against British colonizers.

By the late 1990s, the Mungiki went urban, Mr. Waruinge explained, taking over the city’s minibus trade. Then they diversified into garbage collection, building materials and eventually the protection racket.

“It was beautiful,” Mr. Waruinge said. “We had 500,000 members and millions of shillings coming in every day.”