On the edge of a lonely dirt road that winds through farmland and forest in the eastern Amazon Basin of Brazil stands a simple marble slab. It’s a memorial to a local rainforest defender who was gunned down on his motorcycle, together with his wife, on the site on the morning of May 24, 2011.

Nearly two years later, I stand on the road by a swollen brook, trying to reconstruct the chain of events that led to the brutal deaths of José “Zé Cláudio” Ribeiro da Silva and Maria do Espírito Santo. The afternoon is muggy and overcast, with low-hanging, leaden clouds threatening more rain, raising the prospect of getting stuck out here in the middle of nowhere.

“The gunmen were hiding in the brush over there,” says Maria’s brother-in-law José Maria Gomes Sampaio, who has accompanied me on a bouncy two-hour ride in a 4x4 across flooded plains and fields dotted with dilapidated ranchos and herds of white, hump-backed steers. A wiry man with pleading dark eyes and an Adam’s apple that bobs when he speaks, Sampaio, 49, walked past this very spot only a half-hour before the ambush. “They were already here when I went by,” he says, pointing into the shadows beyond the washed-out bridge that forced the victims to slow their dirt bike to a crawl, putting the couple directly in their gunsight.

The killers evidently knew when the couple would be traveling. In the predawn darkness, they took up positions behind a blind of thicket close by the decrepit bridge. It was a time of day when there would likely be no witnesses. And the shotgun with its spray of buckshot would confound efforts to identify a murder weapon. It was a well-planned operation. Not likely the work of two illiterate, down-and-out men in their early 30s. Certainly not acting on their own, anyway.

From this vantage point at the bottom of a gentle slope, I get an uncanny sense of straddling the very edge of Brazil’s most violent frontier. On the one side of the road, electric-green cattle pastures roll away into the distance, as far as the eye can see. On the other side, colossal castanha and andiroba trees, draped in thick lianas, soar to neck-craning heights, the remnants of a virgin rainforest Zé Cláudio and Maria died trying to defend from the chain saws that had already leveled much of the forest in this part of the Amazon Basin.

Somewhere in the treetops, a toucan yelps. I turn back to inspect the memorial more closely. “They want to do the same thing to me they did to Chico Mendes and Sister Dorothy,” it reads. Prophetic words, spoken by Zé Cláudio at a public gathering six months before he and Maria were gunned down. The inscription is mostly intact, but it’s been vandalized by the impact of two bullets, leaving it fractured.

It has been 25 years since the assassination of Chico Mendes, the rubber tapper who made defense of the Amazon rainforest an international cause célèbre after he was shot dead by the son of a rancher. And it has been nine years since Ohio-born nun Dorothy Stang was killed in similar circumstances. The shattered plaque offers a grim testament to how risky it still is to stand up for the rainforest. Environmental activists in Brazil and around the world continue to pay the ultimate price for their convictions. And their numbers are mounting.

Zé Cláudio and Maria, both in their early 50s at the time of their deaths, had been married for nearly 30 years. For even longer they’d been fighting to protect their lush forestland from illegal loggers, ranchers and the operators of clandestine charcoal pits that reduced magnificent, centuries-old trees to sacks of briquettes. In 1997, they helped succeed in petitioning the federal government to create the Praia Alta-Piranheira agro-forestry settlement, 84 square miles of public land to provide themselves and other family farmers a sustainable living while keeping the forest intact. Its purpose stood in stark contrast to other pursuits that had turned so much of southern Pará, a state in Brazil, into an epicenter of violence and devastation.

But the boundaries of the reserve could hold back neither the bloodletting nor the pillage. Fourteen years after Zé Cláudio and Maria helped found the settlement, its forest cover had shrunk from 80 percent to 20 percent. Speculators snatched up parcels and sold off the timber. They flipped the land to cattlemen and wheeler-dealers looking for a quick buck. They imposed their own brand of frontier justice, tapping when necessary into an abundant pool of underemployed enforcers, or jagunços, from the rough-and-tumble slums of Marabá, Pará’s fourth-largest city, which boasts one of the highest murder rates in Brazil.

Evidently, it was to this reservoir of talent that the enemies of Zé Cláudio and Maria turned in the spring of 2011. Nearly two years later, two out-of-work day laborers—Alberto Lopes do Nascimento, 30, and Lindonjonson Silva Rocha, 31—sat in prison blues in a Marabá courtroom, charged with carrying out the murders with coldblooded calculation. Silva Rocha, named in honor of the 36th president of the United States, happened to be the brother of José Rodrigues Moreira, a rancher whose efforts to acquire land inside the reserve had been repeatedly frustrated by Zé Cláudio and Maria. Moreira, a tightly wound and fervently religious man of 43 with short-cropped auburn hair and pinched brow, was also on trial, accused of ordering the killings.

***

Violence unleashed against green activists is on the rise. London-based rights group Global Witness says more than 700 environmentalists were murdered in the decade that began in 2001. Either because documentation of such crimes is more thorough in Brazil than elsewhere or because its frontier is the most violent—perhaps both—more than half of the global death toll was recorded within its borders. In any event, Brazil is considered the most dangerous country in which to work as an environmentalist today.

Many of the victims of environmentally motivated violence are not your typical placard-waving rabble-rousers, but rather are grass-roots leaders who stand up for their communities when threatened by environmental calamity. “Often these people become involved because they’re fighting for what’s being taken away from them and their communities,” says Jane Cohen, an expert in environmental health at Human Rights Watch in New York City. “They’re especially vulnerable because they usually don’t have a support network, and things can really escalate before their stories get on the national or international radar.”

Worldwide, the most violent years were 2010, when 96 activists were killed, and 2011, the most recent year assessed, when 106 were slain. At that rate, chances are that someone will be killed somewhere on the planet this week for investigating toxic runoff from a gold mine, protesting a mega-dam that will flood communal farmland or trying to shield endangered wildlife from well-armed poachers. Rights advocates warn the upward trend is likely to continue. And because of the spotty quality of reporting, the overall number of killings is likely to be a good bit higher.

“We may be seeing just the tip of a much larger iceberg,” says Bill Kovarik, a communications professor at Radford University in Virginia who tracks cases of abuse perpetrated on green activists. “The world needs to be aware of the people who are dying to save what’s left of the natural environment.”

The underlying cause of the violence appears to be the expanding reach of the global economy into hitherto inaccessible hinterlands. These are regions where governance is shakiest and where traditional, subsistence-oriented communities find themselves up against much more powerful, profit-hungry players.

“It is a well-known paradox that many of the world’s poorest countries are home to the resources that drive the global economy,” reads a 2012 Global Witness report. “Now, as the race to secure access to these resources intensifies, it is poor people and activists who increasingly find themselves in the firing line.”

A Laotian community organizer named Sombath Somphone, 60, vanished from a police checkpoint outside the capital of Vientiane in 2012. His disappearance came after he spoke up for victims of a land-grab scheme that saw village rice fields bulldozed to make way for a foreign-owned rubber plantation.

Francisco Canayong, 64, was president of a Philippine farmers association when he was stabbed to death in 2012. Two months earlier, he had rallied villagers to block a China-bound shipment of chromite ore from an illegal mine that was poisoning local water sources. He and two other activists had also testified that they’d overheard the mine’s boss making plans to kill the trio if they succeeded in shutting down the operation.

In the oak forests of southwestern Mexico, communities are under siege from illegal loggers backed by drug cartels seeking to expand their acreage of opium poppies and marijuana. Entire towns have risen up to torch logging trucks and expel corrupt officials, arming themselves against traffickers and timber poachers. But resistance comes at a high price: Several villagers have been murdered while out collecting mushrooms and firewood in what remains of the forest.

Mexico may be an extreme case, but experts say it points to the connection between the consumption of goods in the rich, industrialized nations and the environmental and human toll in poor nations. Protesters at an Australian-owned mine in Indonesia are threatened and brutalized by government troops. Park guards in Central Africa are ambushed by poachers who slaughter wildlife for tusks and body parts that will ultimately sell as high-priced aphrodisiacs in Asian markets. An uncontacted tribe in Peru faces deadly peril from the encroachment of men and machines exploring for oil that will end up in the pumps of an American gas station. In the eastern Amazon where Zé Cláudio and Maria lived and died, charcoal from illegally cut trees is used to smelt pig iron, a key ingredient in the steel assemblies of cars sold in the United States and Europe.

“There’s a resource that someone wants,” Kovarik says, describing the pattern of events that puts environmental advocates at risk of harm. “People are displaced to get it. They organize and speak up, and their leaders are killed. It’s happening all around the world, and it needs to be investigated.”

The cases are by nature difficult to investigate. Local authorities are often in the pockets of those who have a vested interest in covering up the crime. And the assassinations are likely to involve complicated conspiracies, with instigators distancing themselves through a series of middlemen from the “kill team”—often two men on a fast-moving dirt bike, one driving, the other with a finger on the trigger.

***

Like the murders of Chico Mendes and Dorothy Stang, the deaths of Zé Cláudio and Maria provoked such widespread revulsion that Brazilian officials were forced to act. Bringing the killers to justice came to be seen as an early test of President Dilma Rousseff’s commitment to the rule of law. It also posed a serious challenge to one of her core tenets—that Brazil can remain a bastion of biological and cultural diversity even while exploiting the riches of the Amazon Basin with massive development projects. She dispatched federal agents to investigate.

They had a lot of work to do. After all, José Rodrigues Moreira was but the latest in a long list of people Zé Cláudio and Maria had crossed paths with over the years. As the reserve’s forest cover shrank, the couple had denounced illegal land clearing, unauthorized logging, the illicit buying and selling of parcels, and the charcoal pits that not only devastated woodlands but employed slave labor to do it. And many families on the settlement had turned to ranching themselves after failing to secure credit for more eco-friendly activities such as extracting oils and salves from rainforest nuts and fruits. They came to resent what they saw as the couple’s purist hectoring.

“There was an internal ideological war underway within the settlement,” says Claudelice Silva dos Santos, 31, Zé Cláudio’s youngest sister. I’ve just arrived at the slain couple’s former home, a simple cabin set back in the woods, a few miles from the scene of the crime. Claudelice and several sisters and brothers-in-law are lounging on the front porch, drinking coffee and smoking cigarettes. “The association was divided between those who sought a sustainable alternative to cutting down the forest and those who were willing to partner with outside interests.” The outside interests, she says, are mostly ranchers seeking to extend their pasturelands into the settlement.

The government detectives narrowed their focus in the end to a single line of inquiry, and Moreira and the two alleged triggermen were taken into custody and charged with murder. Oddly, prosecutors did not present what appeared to be evidence of a larger conspiracy. A federal police wiretap recorded Moreira, in hiding after hearing reports that linked him to the murders. In the phone call, he instructed a relative to tell a pair of fellow ranchers to hire an attorney for his defense. Otherwise, he threatened, he would “deliver them all” to authorities. Moreira got his lawyers. The wiretap was not introduced as evidence. The other ranchers were never charged.

The jury in Marabá eventually returned a verdict that astounded everyone in the packed courtroom. The hit men were found guilty; Moreira was absolved and set free. Lawyers on both sides called it “schizophrenic,” contradictory. Without a prime mover—an “intellectual author,” in legal terms—the murders made no sense; neither of the killers had any known connection to the victims, except through Moreira. By the jury’s logic, it was a crime without motive.

The decision left the families of Zé Cláudio and Maria stunned and fearful. Not only were the apparent co-conspirators who Moreira threatened to expose in the wiretapped conversation still on the loose; now Moreira himself was as well. “Sure, we’re afraid,” says Claudelice, her darting eyes probing the nearby forest. The memorial has been shot up, and gunfire has been heard close to the house as well. It’s an intimidation tactic that dates back to the years when Zé Cláudio and Maria were still alive. Back then, she says, Zé Cláudio often maintained a nighttime vigil from the crook of a tree to counter shadowy figures who took potshots at the house that she believes were intended to kill her brother. “Thank God they didn’t succeed...” Claudelice starts to say, then catches herself in mid-sentence at the unintended irony. They did, in fact, succeed all too well. Quickly shifting gears, she adds: “But my brother and his wife fought till the end for an ideal. Who are we if we don’t show the same courage? It was our blood, not just theirs, that was spilled here.”

She and a brother-in-law, Luíz, take me on a short hike back through the woods. Despite the pastureland pressing in from all sides, the 50-acre property feels like a small reserve in its own right, practically all of it intact, virgin rainforest. The decaying leaf litter exudes a spongy dankness underfoot. In ten minutes we reach a towering castanha—a Brazil-nut tree—so wide that it would take at least eight people joined hand to hand to encircle its base. Zé Cláudio had estimated the colossus to be about 600 years old—older than the discovery of the New World itself. Hundreds of similar behemoths inside the reserve have already been toppled to make way for cattle and charcoal.

Rights activists fear the verdict will feed a culture of impunity that reigns in southern Pará and throughout the Brazilian Amazon. Of more than 914 cases of land-related killings over the past 30 years, all but a dozen gunmen have gone scot-free. Only six intellectual authors have served time in prison, amounting to a conviction rate below 2 percent.

***

With receding hairline and bookish eyeglasses, José Batista Gonçalves Afonso, a Catholic Church lawyer who advised the prosecution in the case against Moreira and conspirators, looks more like the priest he studied to be in his youth than the rainforest and human rights crusader he has become, a man who has received multiple death threats. He has helped file an appeal in the case, hoping to bring a new trial against Moreira. “Convicting the boss would have a squelching effect,” he says. “They’ll have to think twice before contracting killers to do their work.”

That’s unlikely to happen any time soon, in Afonso’s view. Brazil has set itself on a course that will see more land conflict, not less, as it seeks to boost commodity exports—minerals, beef and soy—to pay for massive public-works projects and social programs. It could be the government applying eminent domain over indigenous lands to dam a river. Or a rancher illegally clearing land for cattle. Wherever the challenge comes from, there will be push-back from traditional communities. “We see the greatest number of conflicts where the frontier is expanding into the Amazon,” says Afonso, who pledges to stand behind those who resist. “We’re going to confront the loggers, the cattle breeders, the ranchers. We will impede their advance.” It’s a fight he almost seems to welcome. In any case, it’s a fight that’s far from over.