Concerns are mounting for the well-being of British journalist Dom Phillips, who disappeared in Brazil, and Brazilian expert on indigenous affairs, Bruno Pereira. Police found traces of blood in the boat of a suspect arrested in connection with the disappearance on Thursday. The material is now being examined, it said.

The 57-year-old Phillips, who writes regularly for the British newspaper The Guardian as a freelance correspondent, had been researching a book about violence against indigenous people together with Pereira in the Javari Valley near the border with Peru. They have been missing since Sunday. Gold miners, poachers and drug gangs are active in the region. According to indigenous organizations, the men had previously been threatened.

On Wednesday, investigators arrested 41-year-old Amarildo da Costa de Oliveira. According to the police, drugs and cartridges for an assault rifle were found on the suspect during a random check. Witnesses said they saw the man chasing Phillips and Pereira’s boat.

Traces of blood were found in his boat, the police said. “The collected material is en route to Manaus (capital of the state of Amazonas) aboard a Black Hawk helicopter for review.”

Pereira, who works for the government agency for indigenous affairs (Funai), regularly receives threats from illegal loggers and miners trying to trespass on the lands of isolated indigenous groups. In recent years, violence has increased in the region due to the presence of illegal miners, hunters and fishermen.