In the case of the British journalist Dom Phillips, who has been missing in the Amazon since Sunday, and the Brazilian indigenous expert Bruno Pereira, the first arrests were made on Thursday.
The suspect is a fisherman who, according to witnesses, is said to have followed the two with his boat on the day they disappeared.
Forensics found traces of blood in the fisherman's boat, which were sent to the city of Manaus for evaluation.
There it should be determined whether it is human blood.
According to a local indigenous organization, the arrested fisherman fished and poached illegally in the protected areas.
He is said to have threatened Phillips and Pereira two days before they disappeared along with other gunmen.
Pereira, who worked for the Brazilian Indigenous Authority for a long time, had previously been threatened by illegal prospectors, loggers and fishermen for making their activities public.