The release of new photographs of an indigenous group living in extreme isolation in the Amazon rain forest has stirred fresh controversy and new concerns over the fate of the region's so-called uncontacted tribes.

Taken last week from a low-flying aircraft in the far western Brazilian state of Acre (AH-cray), the images depict frightened tribal warriors brandishing spears and arrows as they peer up from palm-thatched huts in the middle of the jungle.

Brazil's indigenous affairs agency, FUNAI, has confirmed the presence of 27 indigenous groups living in extreme isolation in Brazil's vast Amazon region, making it the home of the largest number of uncontacted tribes in the world. (It's possible Brazilian forests may shelter as many as 84 such tribes.) After Brazil, Peru has the second largest number: 15. (Read "Last of the Amazon" in National Geographic magazine.)

The exact meaning of "uncontacted" tribe is a matter of debate, but experts agree that such communities have extremely limited contact with the outside world and that they survive in nearly complete isolation from the global industrial economy. Whatever contact may occur often takes the form of violent clashes—a dialogue of flying bullets in one direction and flying arrows in the other.

At least three such tribes live in isolation in the Upper Envira River region where the overflight took place. Officials from FUNAI told National Geographic that the flight was illegal, and an inquiry is under way to determine what crimes may have been committed by those responsible.

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"The photographs were made without authorization," said Carlos Travasso, head of FUNAI's Department of Isolated Indians, the unit responsible for monitoring and protecting the country's last remaining uncontacted and isolated indigenous communities. "FUNAI is investigating to determine the motives for the flight and to see if we will take [appropriate] measures."

Reached by cell phone while traveling in the Amazon, Travassos said that the overflight, which was organized by an indigenous rights group sponsored by the Catholic Church, violated several government protocols for conducting aerial surveillance of Indians' lands.

"Walking a Razor's Edge"

But a journalist from the Indigenous Missionary Council (CIMI) who was aboard the flight said it was undertaken at the behest of Ashaninka tribal leaders concerned over possible incursions by organized criminal groups and increasingly violent clashes with the uncontacted tribes that inhabit the region.

"It's like walking a razor's edge," CIMI reporter Renato Santana said, describing an atmosphere of tension and fear in the native communities of the Upper Envira River, along the border with Peru.

Villagers in Ashaninka and Kulina settlements told Santana that raids by uncontacted tribal nomads have been on the rise since FUNAI withdrew ground personnel from the region two years ago. The tribesmen rampage through the communities, making off with industrial goods—clothing, axes, machetes, aluminum pots. Villagers also reported several attempted kidnappings of women and children by the bravos, or "wild" Indians, as they commonly call their uncontacted brethren.

"Women are afraid to go into the forest to tend their gardens for fear of abduction," Santana said. "It's a delicate and complex situation."

Officials have long surmised that a heightened presence of drug traffickers and illegal loggers in Peru is pushing uncontacted tribal nomads across the border into Brazil, where they're launching increasingly desperate and brazen attacks on more acculturated, Westernized indigenous settlements. (See a related map from "Peru's Red Gold" in National Geographic magazine.)

Personnel from FUNAI's Upper Envira Ethno-Environmental Protection Front abandoned their outpost following an attack by suspected drug traffickers in 2011.

FUNAI continues to monitor the isolated tribes with yearly overflights, Travassos said. Pilots are under strict orders to maintain a prescribed minimum altitude to avoid terrorizing the uncontacted people. In years past, officials also dropped axes, machetes, and pots from airplanes into the forest, hoping to sate the evident appetite for such commodities and head off possible clashes. The supply flights were discontinued in 2009.

A Mere Invention?

The photographs taken last week depict the same tribe that became a global sensation in 2008, when FUNAI officials released images of Indians in brilliant crimson body paint taking aim with their arrows at a circling aircraft. Government agents said they released those photographs at the time to counter assertions from pro-business politicians that uncontacted tribes were a mere invention of activists determined to halt exploitation of the Amazon's riches.

By chance, the CIMI-organized overflight coincided with a meeting between Brazilian and Peruvian officials in Lima, where the two countries agreed to strengthen cross-border monitoring of the volatile region to protect the uncontacted indigenous groups.

Critics say that weak governance on Peru's side of the lawless border region has served as a convenient excuse for Brazil's failure to do more to protect its most vulnerable indigenous populations. "Brazil cannot only blame Peruvian loggers anymore," says Felipe Milanez, a researcher at the Coimbra University's Center for Social Studies in Portugal. "The state oil company Petrobras is prospecting for gas in the state of Acre, and that is increasing pressure on the tribes as well."

In addition, the Acre state government is reported to have recently built a road straight into the remote region without consulting FUNAI or conducting any prior social or environmental impact assessment. Experts agree that road construction in the Amazon rain forest leads to land speculation, rampant deforestation, and grave risk to indigenous populations.

Pressure is also mounting on another isolated indigenous group far from the border region, in the central Amazonian state of Mato Grosso.

"Enemy!"

FUNAI officials painted a dire picture of the Kawahiva tribe, an isolated indigenous people whose few dozen surviving members live in a state of constant flight from loggers and land speculators who have invaded their territory.

Last year, officials released dramatic video taken by a FUNAI agent of his own near-encounter with the Kawahiva in the depths of the forest. Glimpsing the official, a woman screams "Enemy!" and takes off running. Officials say the Kawahiva view all non-Indians as mortal enemies, having experienced repeated violence at the hands of white intruders.

"The Kawahiva are gravely threatened," said Elias Bigio, former head of the Department of Isolated Indians, who serves as a technical adviser to FUNAI's Madirinha-Juruena Ethno-Environmental Protection Front in Mato Grosso. "There is enormous pressure from loggers, gold prospectors, land speculators. It's an area of conflict and intense timber exploitation."

FUNAI agents assigned to prevent intrusions are also living under threat of violence, Bigio said. "No one goes out in the field alone. We take precautions appropriate to the situation."

FUNAI agents raced against the clock last year to complete court-ordered anthropological studies required to establish a protected reserve for the Indians. But Minister of Justice José Eduardo Cardozo has yet to sign off on demarcating the indigenous territory, and Bigio fears the Indians could disappear altogether if the protections are not definitively implemented.

Brazil's constitution guarantees indigenous populations the right to the undisturbed use of their traditional lands and obliges the federal government to intervene to protect them. To that end, personnel from the Department of Isolated Indians undertake dangerous missions into the deep jungle to document the presence of uncontacted indigenous groups and seal off their lands from the encroachment of outsiders, who often bring violence and epidemic diseases against which the Indians have no immunological defense.

Business interests opposed to the demarcation of the Kawahiva territory have accused FUNAI of "planting Indians" on the land to keep it off-limits to development, Bigio said.

"There are no villains here," said journalist Renato Santana, referring to the strife between the Ashaninka and the uncontacted groups in Acre's border region. "What you have are different societies trying to coexist in a frontier region that is shrinking by the day."

Santana believes the flight gave Ashaninka elders a greater appreciation for the simplicity in which the isolated groups live, and for the precarious nature of their existence. "I have no doubt that the overflight will help avoid violence between the Ashaninka and the isolated tribesmen," he said.

Last Frontier of Human Resistance

Brazilian photographer Lunaé Parracho, who took the photographs for Reuters news agency, offered a more poetic remembrance of the experience. "I felt like I was witnessing the last frontier of human resistance," he wrote in an email from Brazil. "I believe that the existence of these people, and their choice to live in isolation, forces us to think about what is truly important and how far our own society is willing to go in the name of the dollar." (Related: "Rain Forest Warriors: How Indigenous Tribes Protect the Amazon.")