Pieter Hugo for The New York TimesKilling Field The open space between two squatter camps where Farai Kujirichita was beaten to death by vigilantes. 
The mob, desperate for vengeance, had found an unlikely guide to lead them into their dark work. Fifteen-year-old Siphiwe, short, round-faced and reliably smiling, declared, "I know where these criminals live."
Memento A photograph of Farai, which his brother Clemence says was taken just weeks before he was killed. He was a wayward teenager, a bad boy wanting to become a worse boy, and this gave him credibility in the matter of where vicious criminals might be found. A few men lifted him onto their shoulders so that the crowd, already in the hundreds, could see him better. Then an older man, wiser about these things, said to put the boy down. More than likely, they were about to kill someone. No one in the mob ought to be too conspicuous.
Diepsloot, in the northern reaches of Johannesburg, is a settlement of 150,000 people, the majority of them destitute. Crime oversteps even poverty as the most bedeviling affliction, and the night before, a gang of thugs marauded through one of the huge squatter camps in a subdivision called Extension 1. They were a methodical bunch, taking their time, shrewd about where to find stashes of cash amid the pittances, aware also of the police’s reluctance to enter the weave of shacks — the mokhukhus — where the narrow, unlighted pathways can be a fearsome labyrinth. The criminals killed two people, though the churning rumor mill put the number as high as 11.
Siphiwe himself claimed to have been robbed. “They took my cellphone,” he told the others. He was unaccustomed to feeling so important, and he walked cockily at the front of the mob along Thubelihle Street. It was close to noon on a cloudless Saturday morning in late January, the heart of the South African summer. The road abounded with township life: good music playing over bad radios, women pinning laundry to droopy clotheslines, storekeepers brushing aside plump flies in the butchery. People were curious about the mob’s intentions, and some followed along as if dutifully joining a militia.
In a few blocks, the pavement of Thubelihle gave way to hard-packed dirt and stones. A busted pipe had gone unrepaired for months, and the escaping water cut a trough in the ground that now carried a stream of garbage and sewage. The odor was bracing, but there was open air ahead, a large, marshy field that separated Extension 1 from the squatter camp in Extension 2. The mob took an undulating footpath across the terrain, and once it halted, Siphiwe pointed out an empty shack and a locked trailer. These belong to criminals, he said, and the structures were easily torn apart with a few tools and strong hands and then set on fire.
People cheered the crackling of the flames, but this minor demolition was hardly enough to bring their wrath to a cathartic finish. Mob justice is not uncommon in Diepsloot, and most often it involves the swift capture of a supposed criminal, the villain there to beat up, to stone, perhaps even to wrap in a petrol-soaked shroud. But this undertaking was something entirely different. The vigilantes had walked a long distance on a hot day in the uncertain pursuit of unspecified thugs — all on the word of this talkative boy.
The crowd eventually migrated from the cramped lanes of the mokhukhus to a clearing used as a soccer field. A meeting began, and several women from Extension 2 shouted angrily about crime: the shootings and the rapes and how they have to hide their children under the beds. One claimed that criminals hung out in front of a tiny store, what’s known as a spaza shop. The business’s entire stock consisted of two bags of Simba cheese snacks, she said scornfully. “How can you have a spaza that sells only two Simbas?”
Siphiwe led the way, back along the dusty paths between the shacks to the edge of the marshy field. The spaza shop was locked, and though empty of people, it was actually well supplied with soft drinks, biscuits, beer, toiletries and paraffin. The mob nevertheless busted through the walls, and Siphiwe rooted around in a back room, collecting for himself two pairs of sneakers, a Nike track suit and a nylon jacket. The shop was set ablaze, again to the noisy approval of the crowd, though this, too, seemed scant retaliation against murderous thugs. Where were those despicable people?
As the restless mob milled about, a 26-year-old Zimbabwean immigrant named Farai Kujirichita emerged from one of the narrow passageways that led to the field. He was wearing a carefully pressed, lilac-colored shirt and talking into his cellphone. By then, many people were coming and going; his arrival was nothing remarkable. And yet some men from the crowd confronted him.
“Who are you talking to?” they wanted to know. “Who are you warning?”
Then came a more complicated question. “Where are you from?”
Foreigners, especially Zimbabweans, are blamed for much of the crime in Diepsloot. Farai must have decided that it would be safer to lie. He said he was a South African, and his response was in Sepedi, a South African language. This was risky. Nationalities can be easy to surmise, telltale from accents, the style of clothing, the shape of a face, the rhythm in a walk.
Barry Bearak (bearak@nytimes.com) has spent the last three years as a bureau chief for southern Africa for The Times. Editor: Vera Titunik (v.titunik-MagGroup@nytimes.com).