An indigenous rights expert was fatally shot in the chest by an arrow as he approached a tribe.
Rieli Franciscato attempted in vain to shelter behind a vehicle as he and police officers came under fire.
According to a police officer who witnessed the killing the 56-year-old pulled out the arrow, which had hit him in the heart.
The Brazilian man ran 50m before collapsing and dying.
Mr Franciscato had been working in the remote region of Rondônia state in north-western Brazil where he was monitoring a tribe as part of his work for the government’s indigenous agency Funai.
He was an expert in the field, having helped set up Kaninde Ethno-Environmental Defence Association in the 1980s to fight for indigenous rights.
A colleague at the group said the tribe were unable to distinguish between friend and foe.
A policeman who witnessed the incident said in a recording posted on social media: “He cried out, pulled the arrow from his chest, ran 50m (164ft) and collapsed, lifeless.”
Gabriel Uchida, a photojournalist working in the region, told AFP news that the tribe was known as the ‘Cautario River isolated group’.
He said that they were usually “a peaceful group”, but “this time, there were just five armed men – a war party”.
Indigenous rights group Survival International said Mr Franciscato had been called out to investigate in the area after several uncontacted people appeared in recent months.
The group said in a statement: “Most of the forest surrounding the reserve has been destroyed and occupied by ranchers and loggers, who are also targeting the reserve itself.
“Last year numerous fires were started outside and inside the reserve, and this year the ranchers have threatened to burn more of the territory.
“Rieli’s killing, and the appearance of an uncontacted group on the edge of the ranches, are almost certainly a response to the immense pressure they and their forest are under.”
Tensions between indigenous tribes in the Amazon and outsiders have flared in recent years.
Since Jair Bolsonaro took the office of Presidency with a promise to develop the Amazon region, more and more conflicts have broken out between tribes people and loggers.
Survival’s Senior Researcher Sarah Shenker said: “Rieli’s death is a tragic and immeasurable loss for uncontacted tribes, for the forest, and for the fight to stop Brazil’s genocide.
“For decades he refused to accept the violent greed destroying the Amazon rainforest and its best guardians.
“He worked tirelessly to protect the lands of uncontacted tribes from outsiders.
“He dedicated his life to it, working on the front line to combat the illegal invasions by loggers, ranchers and miners who threaten to wipe out the most vulnerable peoples on the planet.
“He didn’t let Bolsonaro’s war on indigenous people and strangling of his budget stop him.”
Bolsonaro has long questioned the need for large indigenous reservations in the rainforest, and has defended opening up protected areas for farming and mining.
The Amazon, the world’s largest rainforest, is home to about 100 isolated tribes.