RIO MARIA, BRAZIL -- After several years of police work on the Amazon frontier, special detective Eder Mauro knows the difference between a seasoned gunman and a rank beginner.

"A professional is not afraid of his victim," Mauro explained. "He walks up to his victim, looks him in the face, chats for a while, then shoots him."

Mauro was dispatched to Rio Maria, a farming town in the north-central Brazilian state of Para -- as far from Rio de Janeiro as Colorado is from Washington -- to investigate the Feb. 2 killing here of Expedito Ribeiro de Souza, president of the local farmworkers' union and a member of Brazil's Communist Party.

Ribeiro was shot three times with a .38-caliber revolver by a lone killer, wearing chaps and a black cowboy hat, on a darkened, muddy street not three blocks from his home.

Ribeiro's death galvanized a network of Brazilian unions, Catholic church groups and American environmentalists seeking to bring a modicum of respect for law to the raucous Amazon frontier. For these organizations, the Ribeiro case echoed the 1988 assassination of Francisco "Chico" Mendes, a unionist who sought to stop the razing of the Amazon rain forest. The triggerman and his father, a cattle rancher who ordered Mendes's death, were convicted in December.

On Tuesday, seven American environmental groups, including the National Audubon Society and the National Wildlife Federation, sent a letter to Brazilian President Fernando Collor de Mello about the Ribeiro killing. "We share Chico Mendes's view that sustainable development in the Amazon depends on an end to impunity for the perpetrators of rural violence all over the interior of Brazil," they wrote.

According to figures compiled by the Pastoral Land Commission, a Catholic human rights group, 65 rural unionists and church activists were murdered in Brazil in 1990.

In response to international concern, federal authorities sent Mauro, a young, aggressive detective, to assist the regular police investigation team in Rio Maria, which consists of one agent and one clerk with no car, no phone and no funds.

After studying details of the crime, Mauro said he had no doubt that the killer was hired by someone else to do the job. Contracting out a murder is so common here that being a paid gunman, or pistoleiro in Portuguese, is an established occupation. But the first bullet that struck Ribeiro came from the back, so Mauro surmised the killer was new to the field.

On Thursday, Mauro arrested Jose Serafim Sales, a former cowhand found hiding on a farm near here. According to Mauro, Sales not only confessed, he also blustered that he had failed in an attempt to kill Ribeiro several days earlier because Ribeiro slipped out of his sight. Sales did not appear to have a prior police record.

Mauro immediately whisked the suspect out of town, fearing that he would be lynched either by farmworkers or by landowners seeking to prevent him from revealing who ordered the murder.

While Chico Mendes's union of rubber tappers fought to prevent ranchers from cutting down swaths of Amazon forest, Ribeiro's union defended squatters and farmworkers seeking land in a region where most of the forest has long been leveled.

During a frontier boom in the 1970s the Brazilian government provided financial incentives to big businesses to develop vast tracts of forest in Para. But small farmers claimed rights to some lands because they had farmed them for many years. Ribeiro's union, formed in 1983, also led land occupations by migrant farmworkers who wanted to settle and farm.

Recently, Ribeiro came to the defense of farm laborers who were held in virtual slavery on nearby ranches. Since 1989, cases in which laborers were "reduced to a condition analagous to slavery," a crime under Brazilian law, surfaced in four counties near here.

Typically, migrant laborers were paid less than they had agreed towork for, and were barred at gunpoint by crew bosses from leaving remote ranches. The workers ran up debts at the company store, which charged a laborer for "everything from the plastic to cover his hut to the file to sharpen his machete," according to one worker's 1990 court testimony.

In October 1989, laborer Jose Pereira Ferreira was shot through the head by crew bosses when he was caught trying to flee from a ranch named Espirito Santo. Pereira survived, but another laborer caught with him was killed. Ribeiro and a parish priest, the Rev. Ricardo Rezende, pressured the ranch owner to pay for Pereira's medical treatment, according to Rezende's diary of the events.

Laborers who fled en masse from the Arizona ranch in March 1990 also sought help from the church and the union. They said in court that crew boss Wilkens Martins Jorge regularly brutalized them and forbade them to leave. Moises Pereira da Silva, a crew leader who worked under Martins, told the court that he had joined in "mercilessly beating" the laborers "because the other bosses did it."

Martins argued in court that the laborers had testified against him because "they all owed money to the firm." Martins and nine of his men served two months in jail but were freed by a judge. The case has never gone to trial.

Ranchers in Para accuse the farmworkers' union of wrecking relations with laborers. Antonio Henrique do Amaral, a lumber businessman who is president of a Para ranchers' association, said in an interview, "The rural laborer is humble, easy to get along with. He is the boss's friend. He doesn't have the courage to invade land.

"What would become of these peons if there was no one to command them?" do Amaral asked.

Ribeiro's 1,300-member union is paralyzed because all its leaders are either dead or in hiding after four other land-related assassinations in Rio Maria in the past year. Carlos Cabral, slated to succeed Ribeiro as president, is leaving the area.

Police issued an arrest warrant last week for the suspected assassin of another unionist, Jose Helio da Silva, killed in December in the state of Pernambuco. Human rights groups have asked the government to provide protection for Cabral, Father Rezende and others who are under threat of death here.

But Eder Mauro observed a simple truth about the Amazon: "When a man is marked to die, there is no security that can save him."