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RIO DE JANEIRO (AP) — Brazil’s federal police said Friday they arrested a man to investigate his possible links with illegal fishing in the area where British journalist Dom Phillips and Indigenous expert Bruno Pereira were killed just over a month ago.

This is the fourth arrest since the beginning of the investigation, and police said in a press conference Friday that the investigation continued, to “see if more people are involved”.

Authorities said they are investigating whether the man arrested on Thursday for presenting fake documents has ties to illegal fishing activities in the Javari Valley Indigenous Territory, which borders Peru and Colombia. The man’s identity is unknown.

The region holds a long conflict between Indigenous tribes and poor fishermen hired to invade the Javari Valley to catch arapaima, turtles and game. Pereira, who was an official of Brazil’s Indigenous affairs bureau, fought these invasions for years and had received multiple threats.

Phillips was traveling with Pereira, researching for a book about how to save the world’s largest rainforest.

Their bodies were found last month after fisherman Amarildo da Costa de Oliveira, nicknamed Pelado, confessed he killed Phillips, 57, and Pereira, before leading police to their remains. Police also arrested Pelado’s brother, fisherman Oseney da Costa de Oliveira.

The case, until now in the hands of state authorities, was transferred to a federal judge this week.