Before Nicolás (Shaco) Flores was killed, deep in the Peruvian rain forest, he had spent decades reaching out to the mysterious people called the Mashco Piro. Flores lived in the Madre de Dios region—a vast jungle surrounded by an even vaster wilderness, frequented mostly by illegal loggers, miners, narco-traffickers, and a few adventurers. For more than a hundred years, the Mashco had lived in almost complete isolation; there were rare sightings, but they were often indistinguishable from backwoods folklore.

Flores, a farmer and a river guide, was a self-appointed conduit between the Mashco and the region’s other indigenous people, who lived mostly in riverside villages. He provided them with food and machetes, and tried to lure them out of the forest. But in 2011, for unclear reasons, the relationship broke down; one afternoon, when the Mashco appeared on the riverbank and beckoned to Shaco, he ignored them. A week later, as he tended his vegetable patch, a bamboo arrow flew out of the forest, piercing his heart. In Peru’s urban centers, the incident generated lurid news stories about savage natives attacking peaceable settlers. After a few days, though, the attention subsided, and life in the Amazonian backwater returned to its usual obscurity.

In the following years, small groups of Mashco began to venture out of the forest, making fleeting appearances to travellers on the Madre de Dios River. A video of one such encounter, which circulated on the Internet, shows a naked Mashco man brandishing a bow and arrow at a boatload of tourists. In another, the same man carries a plastic bottle of soda that he has just been given. Mostly, the Mashco approach outsiders with friendly, if skittish, curiosity, but at times they have raided local settlements to steal food. A few times, they have attacked.

The latest attack, last May, took the life of a twenty-year-old indigenous man, Leonardo Pérez, and this time the news did not subside. People from Pérez’s community wanted revenge, and the governor of Madre de Dios took the opportunity to rail about federal neglect of the area. The government needed to be seen to do something.

A few weeks later, officials announced that they were sending a team to engage with the Mashco, drawn from the Department of Native Isolated People and People in Initial Contact, a recently created sub-office of Peru’s Ministry of Culture. When I spoke to Lorena Prieto Coz, the head of the department, she emphasized that the government preferred not to interfere with isolated indigenous people, but the threat of violence had left no choice. “We didn’t initiate this contact—they did,” she said. “But it’s our responsibility to take charge of the situation.” She told me that an outpost had been set up near where the Mashco appeared, and a team from the department was going soon. She invited me to accompany them.

Map by Luke Shuman Only about a hundred groups of isolated indigenous people are believed to still exist, with more than half of them living in the wilderness that straddles Peru’s border with Brazil. Fiona Watson, the field director of the tribal-people’s-rights group Survival International, told me that the situation was dire for the region’s aislados, as isolated people are called in Spanish. In a cramped London office, Watson laid out satellite maps to show me their territory, small patches in a geography overtaken by commerce: arcs of slash-and-burn farmland; huge expanses where agribusinesses raise cattle and grow soy; mining camps that send minerals to China; migrant boomtowns. Some of the indigenous groups were hemmed in on all sides by mining and logging concessions, both legal and illegal. One tribe in Brazil, the Akuntsu, had been reduced to four members. Near them, a man known to anthropologists only as the Man of the Hole lives in a hollow dug in the forest floor, warding off intruders by firing arrows. He is believed to be the last of his tribe. Unless the trends were halted, Watson said, the Mashco Piro and the other remaining aislados were doomed to extinction—a disquieting echo of the situation of Native Americans in the nineteenth century, as white settlers forced them to retreat or die. “There’s so much at stake here,” Watson said. “These people are as much a part of the rich tapestry of humanity as anyone else, but it’s all going down the drain.”

In the late nineteen-seventies, I made several trips into the Peruvian Amazon, at a time when the jungle was just beginning to open. The governments of Brazil and Peru had recently agreed to build a trans-Amazonian highway, linking the Atlantic and the Pacific, but, aside from some muddy unfinished tracks, the Peruvians’ efforts had been defeated by the “green hell” of the rain forest. The backwoods remained inhabited only by animals and by native people, who in those days were still referred to as “wild Indians.”

On one such trip, in 1977, I travelled up the Río Callería, near the unmarked Brazilian border, with a local guide who spoke a few indigenous dialects. We rode in a long wooden dugout canoe known as a peke-peke, its name derived from the sputtering noise of its motor, a Briggs & Stratton outboard. The motor had a propeller that could be raised—essential in shallow waters. Even so, there were stretches where we were forced to get out and pull the canoe by hand.

One day, after hours on the river with no sign of human habitation, we rounded a bend and saw a dugout canoe, carrying a woman and a child, both with long black hair and naked torsos. At the sight of us, they began screaming and paddling frantically toward the riverbank, where a row of crude shelters sat on a bluff that was cleared of jungle. They shouted a word over and over: pishtaco.

We came ashore cautiously, pulling the boat. The camp had been hastily deserted; I found a fish still roasting on an open fire. The boatman nervously said that we should not continue upriver, or the Indians might attack us. When I asked him about the word the woman and child had shouted, he said that they believed I was a pishtaco, an evil person who had come to steal the oil from their bodies.

Months later, a Peruvian anthropologist explained to me the roots of their fear. The term pishtaco, he speculated, originated in the sixteenth century, when Spanish conquistadors such as Lope de Aguirre began exploring the Amazon. These initial contacts had been so nightmarish as to inspire a cautionary tale that still endured: some of the Spaniards, frustrated that their muskets and cannons rusted so quickly in the jungle humidity, were said to have killed Indians and boiled their bodies in iron pots, then used their fat to grease the metal.

For the next three hundred years, the European settlers and their descendants made few inroads into the Amazon. Then rubber was discovered, and, in the eighteen-seventies, South American rubber barons began to brutalize the jungles of Peru, Colombia, Ecuador, and Brazil. In 1910, the Anglo-Irish diplomat Roger Casement spent three months among rubber traders and the indigenous people who were forced to work for them, and wrote of the abuses he had witnessed. “These [people] are not only murdered, flogged, chained up like wild beasts, hunted far and wide and their dwellings burnt, their wives raped, their children dragged away to slavery and outrage, but are shamelessly swindled into the bargain. These are strong words, but not adequately strong. The condition of things is the most disgraceful, the most lawless, the most inhuman, I believe that exists in the world today.”

Atalaya, on the banks of the Madre de Dios River, is an outpost for adventure tourists, but the recent killing has threatened business in the region. A sign posted in the town warns against contacting or photographing aislados. November 5, 2015. Photograph by Aaron Vincent Elkaim for The New Yorker Photograph by Aaron Vincent Elkaim for The New Yorker

The caucheros, as the rubber barons were called, were daring, ruthless men— the equivalent, in a sense, of modern-day narco-traffickers like El Chapo Guzmán. The most murderously flamboyant of them was probably Carlos Fermín Fitzcarrald. Immortalized in Werner Herzog’s 1982 film “Fitzcarraldo,” he was a man of limitless ambition who bloodily installed himself as Peru’s Rey del Caucho—the Rubber King.

Fitzcarrald was born in 1862, the eldest son of an Irish-American sailor turned trader and his Peruvian wife. By the age of thirty, he had become wealthy enough to build a twenty-five-room riverside mansion, with grounds tended by Chinese gardeners. Seeking to expand his operations, he began looking for an overland route that would connect the Urubamba River with tributaries of the Brazilian Amazon. Using thousands of indigenous conscripts to hack through the jungle, he found that the headwaters were just six miles apart, on either side of a fifteen-hundred-foot peak, and he conceived a railroad that would unite the two river systems. The plan was to sail an iron-plated steamboat, loaded with railroad ties, to the Urubamba’s headwaters. There, native porters would lay a track over the mountain, disassemble the ship, lug its pieces across, and put it back together.

In 1894, he launched an expedition to secure the route, and before setting out he addressed his followers from a balcony of his great house. “Like a good and just father, I take you with me,” he said. “I will reward you with the bounty of the divine mountains that extend from where the Sun rises, and where abundant hunting awaits.”

Their quarry ended up being mostly the Mashco, who then dominated the region. Euclides da Cunha, the Brazilian scientist and explorer, described Fitzcarrald’s meeting with the Mashco’s leader, in which he mustered his armed men to intimidate the natives into coöperating. “The sole response of the Mashco was to inquire what arrows Fitzcarrald carried,” da Cunha wrote. “Smiling, the explorer passed him a bullet from his Winchester.” The Mashco leader examined it, amused, and then took one of his arrows and jabbed it into his own arm, looking on implacably as blood ran out of the wound. “He turned his back on the surprised adventurer, returning to his village with the illusion of superiority,” da Cunha continued. “Half an hour later roughly one hundred Mashcos, including their recalcitrant chief, lay murdered, stretched out on the riverbank.”

It was the beginning of a seemingly endless cycle of destruction. Eight decades after Fitzcarrald’s rampage, I took another trip, on the Madre de Dios, where a gold boom had recently begun. Along the river were small camps of prospectors, who had set up diesel-powered pumps and wooden sluices and were noisily gouging away the riverbanks. Their arrival had clearly unsettled the local Amarakaeri people. The Amarakaeri had once been a sizable warrior tribe, but, by the time I arrived, perhaps five hundred remained, living in rudimentary hamlets, where they survived by fishing with poison and by panning for gold. As for the Mashco, who had lived upriver, there was no sign of them whatsoever. It was as if they had never existed.

The Ministry of Culture’s team gathered a few months ago in Cuzco, high in the Andes, where a van was loaded with provisions. The leader was an anthropologist named Luis Felipe Torres, a slim man in his early thirties with an aquiline face and the unassuming manner of a professional observer. He was joined by Glenn Shepard, an American ethnobotanist. A youthful-looking man of fifty, Shepard had lived for a year in the nineteen-eighties among the Matsigenka people, who shared territory with the Mashco; he had learned their language and returned many times since. Shepard worked at the Emílio Goeldi Museum, an Amazonian-research center in Brazil, but he travelled to Peru frequently as an informal adviser to Torres’s department.

Luis Felipe Torres, an anthropologist with the state’s isolated-tribes team. Photograph by Aaron Vincent Elkaim for The New Yorker Photograph by Aaron Vincent Elkaim for The New Yorker

Soon after we set out, the paved road ended, and we began dropping down the eastern escarpment of the Andes, zigzagging through cloud forest and into the humid lowland jungle. After seven hours, we reached the end of the road, at Atalaya, a huddle of rough wooden houses and bodegas on the upper Madre de Dios River. Atalaya was a destination for adventure tourists; at the shoreline was a jetty lined with brightly painted river canoes. But the recent killing had threatened business in the area. A sign, depicting the silhouette of an aislado with a bow and arrow, announced, “Beware! This is a zone of transit for Isolated Indigenous Peoples. Avoid conflicts: Don’t attempt to contact them. Don’t give them clothes, food, tools, or anything else. Don’t photograph them; they might interpret the camera as a weapon. In the event of incidents, contact the Ministry of Culture.’’