Suspect Confessed to Killing British Journalist Dom Phillips in Amazon, Brazilian Police Say

Police said they are in the process of collecting body parts deep in the forest after Mr. Costa de Oliveira led them to the spot where he said he buried the pair’s remains. Mr. Costa de Oliveira’s brother, Oseney, has also been arrested in connection to the case and a third suspect is under investigation, police said.

The announcement comes after a 10-day search for the men in a case that has prompted international outrage, with the British government and the United Nations joining a host of celebrities to put pressure on Brazilian authorities to step up their search efforts.

“This tragic outcome puts an end to the anguish of not knowing Dom and Bruno’s whereabouts,” said the journalist’s Brazilian wife Alessandra Sampaio in a statement shared on social media.

Tests on a portion of stomach discovered on the banks of the river in the search area last Friday showed it was human, authorities said, but they didn’t confirm whether it belonged to either of the two men. Mr. Phillips and Mr. Pereira were shot, according to a person close to the investigation. Police didn’t say what kind of weapon was used.

Mr. Phillips, a 57-year-old veteran correspondent who had lived in Brazil since 2007 and written regularly for the Guardian newspaper, had been reporting in the remote Javari Valley here near the border with Peru on conflicts between indigenous communities and illegal poachers, fishers, loggers and drug traffickers. He told friends it was the last big trip he needed to make to complete an upcoming book.

Some of Mr. Costa de Oliveira’s family members said in interviews they believe he is innocent, denying he had connections to local gangs. Francisco Freitas, 67, Amarildo’s stepfather, said his stepson had returned from questioning with injuries, leading the family to believe he was tortured during questioning. Police denied the allegations.

“He was a good boy, his mother is beside herself,” said Mr. Freitas.

A swath of largely impenetrable rainforest the size of Portugal, the Javari Valley is home to the largest number of uncontacted indigenous tribes in the world, according to Funai, Brazil’s national indigenous institute.

It has also become the scene of violent crime that environmentalists, tribesmen and officials say has worsened under the administration of President

Jair Bolsonaro.

Located in the heart of South America, its networks of rivers and jungle are hard to patrol—and easy for groups trafficking in illegal fish or timber or cocaine to maneuver in.

While cutting funding for environmental and indigenous-protection agencies, the right-wing populist leader has also eased gun-ownership laws, government figures show, emboldening the region’s criminals, according to indigenous groups, human rights organizations and crime researchers.

Mr. Phillips set out from the riverside community of São Rafael near the border with Peru on June 5 in a small motorboat with Mr. Pereira, a top indigenous expert. They had completed their work and were heading back on the Itaquaí River here to Atalaia do Norte, a town of around 20,000. Then they vanished.

Police said Mr. Costa de Oliveira had confessed to sinking the men’s boat after the crime, removing the motor and weighing it down with mud.

Police said they are investigating several possible motives in the case. Mr. Pereira, 41, a father of three and longtime defender of indigenous communities, had been leading efforts to clamp down on illegal fishing in the region. He had many enemies as a result, including Mr. Costa de Oliveira, who had already threatened to kill him, police and indigenous leaders said.

The day before Mssrs. Phillips and Pereira went missing, indigenous leaders said they also saw the journalist take a photo of Amarildo Costa de Oliveira as he and other men sped past, brandishing their shotguns at them.

“It’s possible they felt threatened,” said Guilherme Torres, one of the police officials leading the investigation from Manaus, the state capital of Amazonas.

There were other dangers. Drug gangs here, which ferry cocaine south from Peru and Colombia, typically dismember their victims’ bodies before dumping them in the river to be eaten by fish, police said. The stomach was only found, Mr. Torres said, because the air caused it to float.

“It’s a region that is out of our control, a corridor for drug trafficking,” said Mr. Torres. Gangs easily recruit poor local fishermen to provide accommodation in remote areas or to transport drugs down the river, he said.

“Fishermen know the river the best,” said Mr. Torres. “Who is better to transport drugs than a fisherman?”

Mr. Bolsonaro’s critics have accused him of belittling Mssrs. Phillips and Pereira by referring to their trip as an “adventure” in recent days. He also said anyone traveling in the region should do so with armed guards, implying the men were reckless to set out alone.

Mr. Bolsonaro has said his administration is committed to combating illegal activity in the region, adding that indigenous communities should be allowed to develop their land rather than live in poverty “like animals in a zoo.”

The region’s criminal organizations are so powerful that they have infiltrated the upper echelons of the region’s politics, said Virgilio Viana, former environment secretary for the state of Amazonas.

Many people disappear in the region, although their cases are rarely investigated or reported nationally, said Mr. Viana, who now leads the Foundation for Amazon Sustainability, a nonprofit based in Manaus.

“There are tens of Doms and Brunos killed every year,” he said.

—Vinod Sreeharsha and Paulo Trevisani contributed to this article.

Write to Luciana Magalhaes at Luciana.Magalhaes@wsj.com and Samantha Pearson at samantha.pearson@wsj.com