Published by the New York Times Sunday Review

Brazil is home to the largest number of uncontacted indigenous communities of any country in the world. Hidden deep in primeval Amazon forests, these groups represent the final frontier of a seemingly inexorable conquest that began with the landing of Portuguese and Spanish navigators on South America’s shores at the start of the 16th century.

The history of Brazil’s Amazon region, as elsewhere in the Americas, abounds with tales of mass death and brutality perpetrated against its native inhabitants. Entire tribes disappeared, many without a trace. Few of these atrocities figured in official accounts; rarely was anyone brought to justice.

But 30 years ago, Brazil took an extraordinary step toward halting the march of this dismal history. It recognized the right of its indigenous people to pursue their traditional ways of life, including remaining apart from modern society. It also recognized that they require intact, pristine forests and rivers to survive as they have since before the arrival of Europeans.

To that end, the government created a special unit inside Funai, its indigenous affairs agency. The unit was charged with protecting those untrammeled lands and ensuring the viability of the indigenous communities living within them. Field agents who previously sought to woo uncontacted tribes from the bush were assigned a new role: to identify the forests where such tribes live and staff outposts to block intrusions that could threaten the well-being of the native populations. The rights of these tribes — and the government’s duty to safeguard them — were enshrined in statutes, international agreements and the country’s 1988 Constitution, which remains the law of the land, at least on paper.

But Brazil is rapidly backpedaling on these commitments. Severe budget cuts to Funai and the forced retirement of its most seasoned backwoods officials have caused the withdrawal of personnel and the closing of nearly one-third of the control posts that guard access to the territories of isolated tribes across the Amazon.

And now Brazilian officials are investigating a massacre believed to have taken place in August on a riverbank deep in the Amazon rain forest. The tribe that suffered this latest atrocity is known as the flecheiros — or Arrow People — a seldom-glimpsed group of hunter-gatherers living in extreme isolation inside the Javari Valley indigenous land, one of a dozen reserves in the Amazon that are home to uncontacted indigenous populations.

I am one of few outsiders to see the flecheiros, albeit fleetingly. I trekked through their land on a 10-week expedition through the far reaches of the Javari Valley with Funai in 2002. Officials and indigenous scouts were on a mission to protect the flecheiros, collecting information from their abandoned campsites and monitoring possible threats to their territory, while seeking to avoid direct contact with them.

Many of these isolated groups splintered off from larger, now-settled communities, and their cultures are familiar to anthropologists and other experts. But the flecheiros have remained so isolated that we do not know what their ethnicity is, what language they speak or what they call themselves.

We do know that they are deft archers who have retreated into one of the Amazon’s most inaccessible redoubts, from which they shun all contact with the outside world. This latest incident goes a long way toward explaining their choice.

It’s also the clearest signal yet that Brazil’s failure to uphold its constitutional obligations carries with it lethal consequences. The Javari Valley indigenous land has been a secure bastion for the flecheiros and 15 other isolated tribal groups by dint of its rugged topography and the policies put in place to protect them. Until now.

The government outpost on the Jandiatuba River, where the massacre is reported to have occurred, was shut down in 2014 because of budget cuts. Since then, alluvial gold dredges, largely manned by itinerant desperados, have penetrated deep upriver. Some of the prospectors also venture out to hunt bushmeat, and it was such a hunting party that came upon the flecheiros. The miners’ reaction apparently was swift and definitive.

Had the checkpoint on the Jandiatuba River remained in operation, it’s likely this terrible encounter could have been avoided. There would be no gold dredges ransacking the Jandiatuba, one of the principal watersheds that sustain the flecheiros, nor hunters stalking game deep in their forests.

As we hiked through the primal homeland of the flecheiros 15 years ago, we found ample evidence that Brazil’s policies were working as intended: towering trees dripping with lianas, crystal-clear streams teeming with fish, an astonishing abundance of birds and wild animals, as well as a profound sense of security the flecheiros must have been enjoying.