Dom Phillips, a British journalist and a Brazilian indigenous specialist, has gone missing in the Amazon rainforest.
Dom Phillips, a regular writer to Brazil's Guardian newspaper, was last seen over the weekend in the Javari district of Amazonas state.
While researching a novel, he was traveling around the region with Bruno Pereira. According to indigenous groups, the two had received threats just days prior.
In the search for Mr Phillips, his family claims that "every second matters."
The two are being sought by the federal police and navy in the isolated area.
According to Brazilian daily O Globo, two fisherman were detained in connection with the men's disappearance on Monday evening. They've been freed since then.
Mr. Phillips, 57, is an Amazon expert who has lived in Brazil for more than a decade.
Mr Pereira is a specialist on Amazonian remote tribes and is now on leave from the government's indigenous affairs agency, Funai.
They'd been in the district for nearly a week and had gone to Jaburu Lake on Friday via boat.
According to two indigenous organizations, they were expected to return to Atalaia do Norte city on Sunday afternoon. The couple, however, never showed up.
Since the two men went missing, their families have voiced their concern.
Mr. Phillips' wife, Alessandra Sampaio, issued a statement saying, "Please respond to the gravity of the moment with prompt steps."
"Every second matters in the bush; every second may be the difference between life and death," she continued.
"Phillips interviewed me for the Guardian in 2017," former Brazilian President Luiz Inacio Lula da Silva tweeted. I hope they are well and safe, and that they will be discovered soon."
The Guardian stated that it was "very worried" about Mr Phillips' location and condition, and that it was "urgently seeking information regarding Mr Phillips' whereabouts and condition."
Violence has erupted in the northwestern Javari area, which is home to over 20 indigenous communities, in recent years, as illicit mining, fishing, and hunting have increased.
In recent years, the government's indigenous affairs agency, Funai, which has a base in the region, has been assaulted multiple times.