Rieli Franciscato, who was 56 years old, was an official for the Brazilian government’s indigenous affairs agency Funai, where he had worked to set up reservations to protect tribes in the country.
Mr Franciscato was hit by an arrow above the heart on Wednesday in the forest near the Uru Eu Wau Wau reservation in the western state of Rondonia as he moved towards a previously uncontacted indigenous group.
“He cried out, pulled the arrow from his chest, ran 50 metres and collapsed, lifeless," a policeman who accompanied the expedition said in an audio clip posted on social media.
The Kaninde Ethno-Environmental Defence Association, which Mr Franciscato helped set up in the 1980s, said the group had no ability to distinguish between a friend or a foe from the outside world.
Gabriel Uchida, a photojournalist in the region, told AFP news agency that the tribe was “a peaceful group” when they were last seen in June.
“This time, there were only five armed men, a war group. That means something must have happened for them to seek 'revenge',” Mr Uchida said.
Indigenous people in Brazil have recently faced increasing threat from invasions by illegal land grabbers, loggers and gold miners, who have been emboldened by far-right president Jair Bolsonaro’s attempts to reduce the size of indigenous reservations in the Amazon.
“We are feeling bewildered by so many deaths in this Brazil that no longer respects indigenous rights,” Ivaneide Cardozo, a co-founder of the Kaninde association, said in a statement.
Sydney Possuelo, a former head of Funai, said Mr Bolsonaro had defunded the agency and left it without the staff needed for security at its isolated posts.
“Rieli was a calm, methodical, soft-spoken man who knew the dangers very well, but he was alone and so he went to ask the police to accompany him,” Mr Possuelo said, adding that the presence of police could have triggered the attack.
The Brazilian presidency did not immediately respond to a request for comment from Reuters.
Additional reporting by Reuters