Blood has been found in a boat owned by a suspect in the disappearance of a British journalist and indigenous affairs expert in a remote region of the Amazon, Brazil's federal police said Thursday.

The discovery comes as Brazil's President said search efforts were continuing for veteran correspondent Dom Phillips and Brazilian researcher Bruno Araújo Pereira, who were first reported missing Sunday in the Javari Valley, in the far western part of Amazonas state. They had reportedly received death threats just days prior.

The suspect was arrested Wednesday and remains in custody, authorities said. Materials collected from the boat have been sent to the Amazonas state capital for forensic analysis.

In a news conference Wednesday, Amazonas state security Secretary-General Carlos Alberto Mansur said the man was arrested after being found in possession of “a lot of drugs” and ammunition used for illegal hunting.

Police have also questioned five other people in connection to the missing pair, he said.

Authorities said Wednesday they were pursuing several lines of investigation, including homicide, and added they still “can't rule out anything.”

Phillips and Pereira were conducting research for a book project on conservation efforts in the region, which authorities have described as “complicated” and “dangerous,” and known to harbor illegal miners, loggers, and international drug dealers.

'Slow response'

In an open letter on Thursday, editors from major global news outlets, including The Guardian and The Washington Post -- two newspapers Phillips contributed to -- called on Brazilian President Jair Bolsonaro to “urgently step up and fully resource the effort to locate Dom and Bruno.”

“We are now very concerned by reports from Brazil that search and rescue efforts so far have been minimally resourced, with national authorities slow to offer more than very limited assistance,” the letter added.

Phillips' sister told CNN “It was only on Tuesday evening we saw an official document saying that the army was to be deployed.

“It's a very slow response and it signifies something sinister to the world about information coming out of the Amazon.”

Brazilian authorities said Wednesday that a total of 250 men, two helicopters, three drones and 16 boats have been deployed for the search and rescue operation.

In a tweet Thursday, President Bolsanaro confirmed the search was ongoing.

“Our three forces [Army, Navy, and Air Force], as well as our Ministry of Foreign Relations, the Federal Police, among others, have been working intensely since Monday (6th) in the search for Bruno Pereira and Dom Phillips,” Bolsonaro said.

“We want to solve [the case] and bring comfort to the families,” (CNN)