Dom Phillips disappeared on Sunday morning while on a two-day boat trip through the Javari region of Brazil’s Amazonas state where he was reporting for a book he is writing about conservation.
The 47-year-old, who is a regular contributor to the Guardian, had been travelling with Indigenous expert Bruno Araujo Pereira when the pair went missing in a part of the Amazon rainforest which is close to the Peruvian border and a notorious drug-trafficking route.
Guilherme Torres, the head of the interior department of Amazonas state's civil police, told Reuters in an interview on Tuesday that police had opened a criminal investigation and interviewed four witnesses while also seeking to locate Mr Phillips and Mr Pereira.
Torres said Pereira had recently received a threatening letter from a local fisherman who police were trying to locate.
He said his colleagues had interviewed two fishermen as witnesses on Monday, with two more quizzed on Tuesday. The first two witnesses had not provided any useful information, and Torres had no details as yet about the second two interviews.
“We are indeed working with the hypothesis that a crime might have occurred, but there is another, much larger possibility, that they're lost," Torres said. "Now, our priority is to find them alive, especially in these first hours. In parallel, a criminal probe has been opened to see if there was some crime committed."
Mr Phillips’ sister Sian said she was “desperately worried” amid concerns local authorities weren’t deploying enough resources to the search.
She said: “Time is crucial, because we know there are situations where they could be abducted – but there’s also situations where they had a mechanical fault and they’re stuck and they have short supplies.”
She called her brother “a bright star” after his work highlighted the destruction of the world’s largest tropical rainforest.
“So it’s crucial that [Brazilian authorities] are searching with all the equipment and local knowledge and resources that the army has,” she added.
Sian Phillips appeals for urgency from Brazilian authorities in the search for brother Dom Phillips, journalist missing in the Amazon since Sunday, please keep up the pressure!@FCDOGovUK@trussliz@BrazilEmbassyUK@mhopkinsfco @marcolonghi4dn@fyldeca@jamescleverly pic.twitter.com/5KruRDOKjK
— Paul Sherwood (@paulsherwood6) June 7, 2022
Brazil's navy and army have both dispatched search teams in boats and helicopters to the area, with support from federal and state police.
The men had received death threats in the days prior to their disappearance, according to the Coordination of the Union of Indigenous Peoples of the Javari Valley (UNIJAV).
The threatto Mr Pereira read: “We know who you are, and we’ll find you to settle the score” , according to the newspaper O Globo.
Mr Phillips’ wife, Alessandra Sampaio, who lives him in the north-eastern city of Salvador, made another tearful plea on camera appealing for help to find them.
“I want to make an appeal to the federal government and the relevant organs to intensify their search efforts, because we still have some hope of finding them.
“Even if I don’t find the love of my life alive, they must be found, please – intensify the search,” she begged.
The Javari valley region has experienced repeated shootings between hunters, fishermen and official security agents, who have a permanent base in the area, which has the world’s largest population of uncontacted Indigenous people.
It is also a major route for cocaine produced on the Peruvian side of the border, then smuggled into Brazil to supply local cities or to be shipped to Europe.