The killing of Gerson de Jesus Bispo on Thursday shocked human rights officials who have struggled to control rogue police accused of torture and summary executions in Latin America's largest country.
Bispo was the second person killed after speaking to Asma Jahangir, the United Nations official investigating extrajudicial and summary executions.
"It is a clear challenge to demonstrate that, despite the pledge of the president to protect people, they (the death squads) are untouchable," said Humans Rights Secretary Nilmario Miranda.
The United Nations deplored the slaying and said it was trying to get more information about the incident in Santo Antonio de Jesus in northern Brazil.
"We learned with great consternation this morning of the reported killing of a second person who had spoken to the special rapporteur on extrajudicial executions," said Jose Diaz, spokesman for the U.N. High Commissioner for Human Rights in Geneva.
Just a day earlier, President Luiz Inacio Lula da Silva had told Jahangir that "it was an absolute necessity to protect those who had the courage to seek justice."
The U.N. representative was in Brazil under an official invitation to investigate police involvement in arbitrary executions. She said she had discovered "a gruesome picture which is not worthy of a fit, democratic Brazil."
The gunmen apparently had already sent a signal of their discomfort with the mission of the U.N. official — another person Jahandir had interviewed, Flavio Manoel da Silva, was mysteriously killed on Sept. 27.
The government ordered the Federal Police to investigate Bispo's killing in Santo Antonio de Jesus in Bahia state, some 125 miles from the state capital, Salvador.
The summoning of federal police is rare in Brazil and usually means the state has lost control of a situation. Miranda said a dozen people were already under protection as part of the investigation.
Santo Antonio de Jesus, a town with a population of less than 10,000, appeared calm on Thursday evening.
Gerson de Jesus Bispo was investigating the killing of his own brother and a friend. He met with Jahangir on Sept. 20.
A report by a human rights council in Bahia state said death squads began operating in Santo Antonio de Jesus in 2001, after a fireworks factory exploded and killed 64 people.
The report said that since then, authorities recorded "42 missing people, probably all dead. ... All those missing are youth, with ages between 16-26, generally poor and black."
The report did not say if there is a link between the factory fire and the disappearances.
Summary executions, some with police involvement, have become a commonplace in Brazil in recent years.
Ten years ago, human rights defenders were shocked by two massacres in Rio de Janeiro, Brazil's main tourist city. Eight street children were killed while they lay sleeping outside a church known as A Candelaria. A month later, 21 residents of a poor neighborhood were killed by heavily armed men.
Human rights watchdog Amnesty International said recently that only two of the 50 policemen accused of involvement in that massacre are in prison. According to Amnesty, thousands of Brazilians died in confrontations with Brazilian police last year. Torture was described as widespread.