UN special rapporteur on human rights Hina Jilani said authorities had to catch all those behind the February killing of 73-year-old rainforest activist Dorothy Stang, which drew world attention to Amazon land battles between peasants and landholders.
"We look forward to the government taking measures to catch the perpetrators," Jilani told reporters on the first day of a 15-day visit to Brazil. "I have communicated my concern to the government and urged it to ensure impunity does not prevail."
Police have so far charged five men directly involved in the contract killing near the jungle town of Anapu, Para state. Human rights activists and members of Brazil's government suspect those charged are only low-level actors in a wider plot by powerful landholders.
A naturalized Brazilian born in Dayton, Ohio, Stang spent 30 years protecting the rainforest and peasant farmers. She made enemies among loggers and ranchers who coveted areas she defended.
Two men accused of firing the shots that killed Stang are to face trial in Belem, Para's capital, on Friday. Three others will face trial later.
Brazilian Justice Minister Marcio Thomaz Bastos has vowed to continue the Stang investigation to track down landholders who may have ordered the killing. The outcome of that inquiry may raise the number of individuals held responsible for the crime.
Jilani, special representative of UN Secretary-General Kofi Annan on human rights defenders, is in Brazil to investigate government efforts to guarantee the safety of activists and allow them to denounce abuses.
President Luiz Inacio Lula da Silva promised a crackdown on extra-judicial Amazon killings after Stang's murder.
The number of rural workers and activists killed in Brazil between January and August rose to 28, one more than during the same period of 2004, according to the Pastoral Land Commission, a human rights group linked to the Catholic Church.
Half of all deaths were in Para, one of five states Jilani will visit.