Brazilian detectives have said they are investigating whether it is animal or human blood.
The development comes days after Dom Phillips, a freelance reporter who has written for the Guardian, and Bruno Pereira, who has worked for the federal indigenous agency Funai, disappeared.
The pair had been on a reporting trip in Javari Valley, an area of jungle close to Brazil’s border with Colombia and Peru.
Investigators believe their disappearance could be linked to organised crime, as Mr Pereira had recently been threatened by fishermen who poach protected species.
“The principal criminal hypothesis at this point is that the people involved, and their motive, was related to illegal fishing and poaching activities in indigenous territories,” a federal police officer told Reuters.
Amarildo da Costa, a fisherman from the town of Atalaia do Norte, has been detained by the authorities and charged with possession of restricted ammunition. They say he was among the last people to see Mr Phillips and Mr Pereira.
His lawyer has denied that he is linked in any way to the men’s disappearance, insisting that his client only fished legally.
The missing pair’s families have called on the Brazilian government to do everything it can to help locate the men.
Celebrities including the footballer Pelé have echoed these pleas. “The fight for the preservation of the Amazon Forest and of the indigenous groups belongs to all of us,” the 81-year-old said.
“I am moved by the disappearance of Dom Phillips and Bruno Ferreira, who dedicate their lives to this cause. I join the many voices that make the appeal to intensify the search,” he added.
“Let’s pray to God that they are still alive,” Jair Bolsonaro, the Brazilian president, noting that the “those chances fade” with each passing day.
Currently, 300 hundred people, two planes and 20 boats are taking part in the search operation, according to the Brazilian justice minister Anderson Torres.