Photojournalist Gabriel Uchida, who also witnessed the attack, said the tribe — known as the Cautario River isolated group — were normally peaceful; but "this time, there were just five armed men - a war party".
Colleagues believe Franciscato may have been mistaken by the tribe as one of the invaders he was trying to protect them from.
Conservationists have long been fighting against the controversial policies of President Jair Bolsonaro, who has rolled back protections for indigenous groups in the Amazon rainforest.
In 2018 an American Christian missionary, John Allen Chau, was also shot and killed with an arrow shot by another self-isolated tribe, the Sentinelese, after illegally traveling to their island off the coast of India.
Unlike Franciscato however, who vigorously defended the Amazonian tribes from contact, Chau was actively trying to convert the Sentineli to Christianity.