Dom Phillips went missing on Sunday with Bruno Araujo Pereira, a local indigenous expert and former government official whose job was to protect uncontacted tribes in Brazil.

Officers have interviewed at least four witnesses believed to be among the last to have seen Mr Phillips, a freelancer who has written for The Guardian, The Washington Post, The New York Times and other publications.

Guilherme Torres, the head of the interior department of Amazonas state's civil police, told Reuters news agency that Mr Pereira had recently received a threatening letter from a local fisherman, whom police were trying to locate.

He said his colleagues had interviewed two fishermen as witnesses on Monday, with two more questioned on Tuesday.

The first two witnesses had not provided any useful information, and Mr 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," he 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."

The Brazilian navy and army have both dispatched search teams in boats and helicopters in an attempt to find the pair and are being supported by federal and state police officers.

Mr Phillips, 57, and his guide went missing during a reporting trip in the Javari Valley.

On the border of Peru, the vast region is home to the world's biggest number of uncontacted indigenous people and is
threatened by illegal miners, loggers, hunters, and coca-growing gangs who make the raw material for cocaine.

Mr Torres said he could not rule out that their disappearance was linked to the gangs operating in the lawless region.

The pair had been with an indigenous patrol that was threatened by armed men on Saturday, according to representatives from the Union of Indigenous Peoples of the Javari Valley, which first announced their disappearance.

They were said to have recorded the confrontation on a mobile phone.

The disappearance of the two men, who both had years of experience working in the complex and inhospitable Amazon rainforest, prompted global concern from human rights groups, environmentalists, politicians and press freedom advocates.

Mr Phillips' wife, Alessandra Sampaio, urged authorities to intensify their search efforts in an emotional television interview, "because we still have a little hope of finding them."

"Even if I don't find the love of my life alive, they have to be found, please," she added.

On Monday, his sister Sian Phillips told Sky News she was concerned there is illegal logging and drug trafficking in the area where he disappeared.

"I'm very anxious. I'm desperately worried. It's your worst fear," Ms Phillips said.

"We need everything thrown at this. We want UK officials to put pressure on the Brazilian authorities to act."

Mr Pereira's family issued a statement calling for a robust search operation, adding that "we are also very hopeful that there was an accident with the boat and that they are waiting for help".

President Jair Bolsonaro said in a television interview on Tuesday that the two men "were on an adventure that is not recommended".

"It could be an accident, it could be that they were executed, anything could have happened," he said. "I hope, and
we pray to God, that they are found soon."

The Brazilian leader had faced tough questioning from Mr Phillips at news conferences about policies that have weakened the country's environmental law enforcement.

Indigenous patrols have repeatedly complained about weaker environmental law enforcement since Mr Bolsonaro came into power and call for fewer restrictions on tribal lands in the Javari Valley.

They also regularly clash with illegal miners and hunters in the region.

Mr Phillips had been researching a book about the Amazon and its environmental defenders.