RIO DE JANEIRO — The Brazilian authorities said on Tuesday that they had arrested a second man in the disappearance of a British journalist and a Brazilian expert on Indigenous people deep in the Amazon, confirming that their efforts were shifting from a search-and-rescue operation to a homicide investigation.
The Brazilian federal police said they had arrested Oseney da Costa de Oliveira, the brother of a fisherman who was detained last week. Witnesses saw Mr. de Oliveira and his brother, Amarildo da Costa de Oliveira, in a boat just behind the missing men shortly before they were last seen on a remote river more than a week ago, according to investigative documents viewed by The New York Times. The police said that both suspects were being held in a murder investigation, but they had not been charged.
The missing men — Dom Phillips, 57, a freelance writer for the British news organization The Guardian, and Bruno Araújo Pereira, 41, an expert who worked extensively in the region — were last seen on June 5 while traveling in a boat on the Itaquaí River in the northern Brazilian state of Amazonas, near the borders with Peru and Colombia. Mr. Phillips was reporting on patrol teams that Mr. Pereira had helped create to crack down on illegal fishing and hunting, an initiative that had led to threats against Mr. Pereira.
Their disappearance has transfixed Brazil, drawn search crews from the military and Indigenous groups to scour a remote section of the rainforest and forced President Jair Bolsonaro to defend his government’s response in front of other world leaders at an international summit in Los Angeles last week.
On Monday, Univaja, a local Indigenous association that worked with Mr. Pereira and helped organize the search operations, said those searches would soon end. That news, along with the police’s confirmation of a murder investigation, underscored that the case was no longer focused on finding Mr. Phillips and Mr. Pereira alive, but rather on figuring out who may have killed them.
“The chances that they are found alive are now practically zero,” said Manuel Chorimpa Marubo, a local Indigenous leader who has aided the searches with Univaja. “The hope now is finding the bodies.”
On Tuesday, Brazil’s federal police sent a report to the nation’s Supreme Court summarizing their evidence. In the report, which was viewed by The Times, the police said that just after a local resident saw Mr. Phillips and Mr. Pereira on the morning of June 5, the witness saw Oseney da Costa de Oliveira rowing a small boat in the river. The witness then helped tow Mr. de Oliveira to another boat, where his brother, Amarildo, was waiting.
The police said last week that witnesses had seen Amarildo traveling in his green speedboat with a visible Nike symbol just behind Mr. Phillips and Mr. Pereira that same day.
The day before, Univaja said, Amarildo had threatened a group that included Mr. Phillips and Mr. Pereira by showing them a gun. The federal police said in the report on Tuesday that Amarildo had previously threatened Mr. Pereira and had fired a gun at the building of a government agency tasked with protecting Indigenous groups, according to the report. The police have said they are testing blood found on Amarildo’s boat.
The Brazilian authorities said on Sunday that items belonging to Mr. Phillips and Mr. Pereira, including clothing and a backpack, had been found in the area where they disappeared. They said they had also found “apparently human organic matter” near where the men were last seen and were testing it for DNA. The organic matter is a human stomach, the federal police report viewed by The Times said.
Illegal activity in the region, which includes an Indigenous reserve roughly the size of Maine, has flourished in recent years as government authorities have retreated, officials at Univaja said. Mr. Bolsonaro, an unabashed supporter of developing the Amazon, has dismantled environmental protections and slashed the budgets of the agencies responsible for policing the forest.