On the morning of May 24, Brazilian environmentalist José Cláudio Ribeiro da Silva and his wife, Maria, went for a motorcycle ride through the Amazon rain forest. For nearly three decades, the couple had supported themselves by extracting cooking oil and medicinal ingredients from Brazil nuts. When Zé, as Da Silva's friends called him, learned that ranchers were trying to log public land illegally, he and Maria fought back: They orchestrated protests and alerted the federal authorities. Soon the pair received death threats ("Get ready to be silenced forever") and heard that a $5,000 bounty had been placed on their heads. Zé and Maria had learned to vary their travel routes, but someone must have known where they were headed that morning. When the couple slowed for an unsteady bridge deep in the jungle, they were struck by a 15-bullet barrage. They both died instantly. Before fleeing, the gunmen paused to cut off Zé's ear, a message to investigators that the killings were a hired hit.

According to a local watchdog group known as CPT, more than 1,500 Brazilians have been killed trying to protect the Amazon rain forest over the past 25 years, and some 2,000 more have received death threats. Since the couple died, at least five more people have been assassinated. One activist was shot in front of his wife and children. It's safe to say that Brazil is now the world leader in endangered environmentalists.

"Many people from the logging and mining companies think the only way to solve problems is by killing the people who defend the forest," says Felício Pontes, a federal prosecutor in Brazil and one of Zé and Maria's close friends. "But surely there is a way to develop the Amazon without simply cutting down all the trees."

For decades, Brazil has struggled with how to manage the staggering natural resources of the Amazon. The region, which is nearly the size of the contiguous United States, shelters 10 percent of the Earth's known species. The Nature Conservancy, a nonprofit environmental group, estimates that an average four-square-mile plot in the Amazon contains 750 species of trees, 400 species of birds, and 150 species of butterflies. The rich soil means there's also huge financial potential for farming and livestock. In the 1970s, the country's dictatorship inaugurated a policy known as integrar para não entregar (­"Occupy it, or risk losing it"), and a frenetic landgrab ensued. The new land barons seized vast tracts of forest, which they promptly clear-cut. The timber was sold to foreign countries or burned to make charcoal, and the land was planted with soybeans or populated with cattle.

This, in part, has led to Brazil's emergence as a global economic power. It is already the world's largest exporter of beef and the second-largest supplier of soybeans. The country is expected to soon pass France and Britain to become the world's fifth-largest economy. But the World Wildlife Fund reports that by 2030, more than half of the Amazon rain forest could be gone. And in states like Pará, where Zé and Maria lived, the murder rate has risen more than 400 percent since 2000 – to 40 per 100,000 residents. Magnates buy off local politicians and policemen, and kill anyone who challenges their agricultural practices.

In July 2011, public outcry over the violence forced the Brazilian government to announce that it would provide protection to approximately 130 threatened activists, and at the end of September, four months after Zé and Maria's deaths, the country's Council of Justice unveiled plans to create a special task force to investigate the killing of activists. According to federal prosecutor Pontes, however, only five of the 40 threatened activists in Pará have received any government protection. A spokeswoman for the Council of Justice said that her office has actually reduced the number of unsolved murders from nearly 700 to 74. ("The court figured that a dispute over land was not the main reason behind many of the murders," she said.)

Brent Rushforth, a lawyer in Washington, D.C., who in 2005 helped Brazil prosecute the killers of a 73-year-old American nun and rain forest activist named Dorothy Stang, blames flaws in the country's legal system for the inefficacy of these programs. "Brazilian criminal law is a stew that's made up of common law, Napoleonic code, and Brazilian salsa," Rushforth says. "The Amazon is so vast and completely unfettered to any true rule of law." Rushforth says that to find the men who killed Stang, the Brazilian government had to send its army into the rain forest – and it did so only in response to international pressure.

Many believe that the Brazilian legislature operates under the undue influence of the ranchers, who have the money to win favor. Proof of this seemed to arrive on the day Zé and Maria died, when the Brazilian congress voted to amend the country's forest code, granting amnesty to ranchers who have illegally cleared land while also reducing the percentage of holdings that they're required to preserve. As an ecologist with Greenpeace put it, "Brazil woke up to the news of the murders of two leading environmental activists, and it's going to bed with the murder of the forest code."

Meanwhile, the killings and death threats continue. Recently, gunshots were fired outside the home of Maria's sister, Laísa, a schoolteacher who has been publicizing the murder of her relatives. When Laísa's husband ventured outside, he found the family dog wounded. "People wonder if the government is doing enough," says Pontes. "How can it possibly be enough? If it were, we wouldn't have these killings."