Prosecutors in Brazil are investigating reports that illegal gold miners allegedly massacred up to 10 members of a remote, uncontacted tribe in the Brazilian Amazon.

The alleged attack, which is said to have taken place last month, occurred along the River Jandiatuba in western Brazil, and may have gone forever unnoticed by the outside world, were it not for the the miners being overheard boasting about the killings in a nearby town afterwards.

"It was crude bar talk," Leila Silvia Burger Sotto-Maior, a coordinator for Brazil's indigenous affairs agency, FUNAI, told The New York Times.

"They even bragged about cutting up the bodies and throwing them in the river."

Two of the gold miners – known as garimpeiros – have been arrested and taken to the city of Tabatinga to provide testimony about the incident, which, if confirmed, is feared to have wiped out one-fifth of the entire tribe, including women and children.

The Amazon region where the massacre is said to have happened is called the Javari Valley (aka the Uncontacted Frontier), bordered by Peru, Brazil, and Bolivia.

This area is thought to contain more uncontacted tribes than anywhere else on the planet, with an estimated 2,000 individuals across up to 20 isolated groups.

This tribe is known locally as Fleicheros, or "the ones who throw arrows", but as they're an isolated people with little or no contact with the outside world, not an awful lot is actually recorded about them – something which makes attacks like this hard to investigate.

"We are following up, but the territories are big and access is limited," the prosecutor in charge of the case, Pablo Luz de Beltrand, told The New York Times.

"These tribes are uncontacted – even FUNAI has only sporadic information about them. So it's difficult work that requires all government departments working together."

The region has a history of indigenous peoples being threatened by the illegal encroachment of miners, which commentators say is a direct result of agencies like FUNAI having their funding slashed under the government of the current Brazilian president, Michel Temer.

Temer provoked uproar less than a month ago after attempting to abolish protections shielding a vast reserve of Amazon rainforest from mining interests.

That move – dubbed "the biggest attack on the Amazon of the last 50 years" – has been temporarily foiled by a federal court, but if the action goes ahead, it could increase the dangers and displacement indigenous peoples in the rainforest are clearly already facing from illegal miners.

"If these [massacre] reports are confirmed, President Temer and his government bear a heavy responsibility for this genocidal attack," Stephen Corry, the director of tribal rights organisation Survival, said in a statement.

"The slashing of FUNAI's funds has left dozens of uncontacted tribes defenceless against thousands of invaders – gold miners, ranchers, and loggers – who are desperate to steal and ransack their lands."

In this instance, it's unclear whether the evidence will be sufficient to prosecute the alleged killers.

An audio recording of the boasting in a bar does exist, however, and the miners are reported to have collected tools and jewellery from the victims, which could corroborate the prosecution's case.

Whether those items alone will be enough to punish this alleged horror is unknown, but it's clear that more grave injustices could occur if the sanctity of these isolated people's lands isn't defended by modern forces surrounding them.

"All these tribes should have had their lands properly recognised and protected years ago," Corry said.

"[T]he government's open support for those who want to open up indigenous territories is utterly shameful, and is setting indigenous rights in Brazil back decades."