The Brazilian authorities say they are hopeful of finding the pair alive but do not exclude any outcome, including that of homicide, in a region where trafficking is rife.
Traces of blood have been found on the boat of a suspect arrested in connection with the disappearance of a British journalist and a Brazilian indigenous expert in the Amazon.
Dom Phillips, 57, a contributor to The Guardian newspaper, and Bruno Pereira, 41, a specialist in indigenous peoples, were reported missing on Sunday after they ventured into the middle of the Amazon rainforest.
"Traces of blood were found on the boat of Amarildo da Costa de Oliveira, 41," Brazil police said in a statement on Thursday, adding that the suspect known as "Pelado" was arrested on Tuesday.
"The material collected is on its way to Manaus," the capital of the Amazonas state, for expert analysis, the statement added.
It was accompanied by images of investigators taking photos of what appeared to be a small bloodstain on a blue tarp inside a motorboat with peeling paint.
Calls grow to intensify the search
High-profile personalities and environmental and human rights groups have rallied to the cause, urging President Jair Bolsonaro to step up the search.
"Where is Dom Phillips? Where is Bruno Pereira?" asked the journalist's sister, Sian Phillips, in a statement to the media during a gathering of around 30 people in front of Brazil's embassy in London.
"We want the UK authorities to put pressure on the Brazilian government," she added, before she and other family members were received by the ambassador.
She blamed the Brazilian authorities for delaying the search but said they "all have hope" that the pair will be found.
Bolsonaro, who was attending the Summit of the Americas in Los Angeles, said: "Let's pray to God that they are still alive." But, he added, "With each day that passes, those chances fade."
He had drawn criticism in past days for appearing to blame the missing men, saying they had undertaken an "unadvisable adventure".
Phillips and Pereira went missing in the Javari Valley in Amazonas state, located in the west of the Amazon basin, near Peru.
Witnesses said they saw the suspect speeding by in a boat going in the same direction as Phillips and Pereira when they were last seen. Police said the man had been arrested for carrying unlicensed caliber ammunition and drugs.
The remote region is experiencing an escalation in armed violence due to the presence of miners, gold diggers, poachers and drug traffickers.