His death followed the Feb. 12 killing of prominent US human rights and environmental activist Dorothy Stang as she set up a government peasant farming reserve in the state of Para and defended it from illegal loggers and ranchers.

"He had received death threats for some time," said Luis Henrique dos Santos, head of the Tingua reserve and a federal employee. "We are working against illegal palm cutting and this upset a lot of people."

President Luiz Inacio Lula da Silva launched Brazil's biggest ever crackdown on crime in the Amazon rain forest after Stang's murder caused world outrage at death squads used by illegal loggers and ranchers to invade jungle areas.

Stang had received repeated death threats.

Brazil's federal environmental agency IBAMA, which operates the Tingua reserve, has asked for federal police protection for its workers in the 96 square-mile (249 sq-km) area of rolling, rain forested hills.

"This business of shutting up ecologists and environmentalists with violence, it's not going to stop," said Edson Bedin, head of IBAMA in Rio de Janeiro state. "Threats against agents, workers have become routine."

Ribeiro was a member of a non-profit environmental organization that helped set up the park in 1989. He worked to defend it from people harvesting palm trees for heart of palm, a gastronomic delicacy, and trapping tropical birds and animals for illegal sale.

Police in Rio de Janeiro state said they would make the hunt for Riberio's killers a priority.

Police have found two gunmen and an intermediary suspected of planning and carrying out Stang's killing. They are searching for a rancher who is said to have paid $19,300 for her murder.

Brazil has already deforested 97 percent of its Atlantic rainforest, which runs along its coastline and was once a third the size of Brazil's Amazon jungle.