The wife of a British journalist who disappeared in the Brazilian Amazon has urged the country's authorities to do more to find "the love of my life".
Dom Phillips, 57, went missing in a remote part of the Amazon on Sunday along with Brazilian indigenous expert Bruno Araújo Pereira.
In an tearful video, Mr Phillips's wife Alessandra Sampaio said she still had "some small hope" of finding them.
Brazilian police have opened a criminal investigation, Reuters reported.
The news agency said police had interviewed at least four witnesses believed to be among the last to have seen the journalist and indigenous expert in the Javari Valley.
Guilherme Torres, the head of the interior department of Amazonas state's civil police, told Reuters Mr Pereira had recently received a threatening letter from a local fisherman who police were now trying to locate.
Two days after the pair's disappearance, Mr Phillips's distraught wife released a video message.
"I want to make an appeal to the government to intensify the search. We still have some small hope of finding them. Even if I don't find the love of my life alive, please find them," she said, choking back tears.
Mr Phillips has been living in Brazil for more than a decade. He is a long-time contributor to the Guardian newspaper in the UK as well as other publications such as the Financial Times and the Washington Post.
He has written extensively about the threats facing the Amazon, including how cattle farming is fuelling an environmental crisis and how illegal gold miners encroach on indigenous territory.
The veteran journalist had been researching a book when he went missing.
Mr Pereira, 41, is an expert on isolated tribes in the Amazon and is currently on leave from his post with the government's indigenous affairs agency Funai.
The Brazilian government expressed its "grave concern" at the pair's disappearance, and said police were taking "all possible measures to find (the men) as quickly as possible."
But there has been criticism of the small number of police and navy personnel deployed in what is a vast expanse of rainforest crisscrossed by many rivers.
Brazil's President Jair Bolsonaro was also criticised for appearing to blame the missing men, both of whom have years of experience working in the Amazon rainforest basin.
"Two people in a boat in a region like that, completely wild - it's an unadvisable adventure. Anything can happen," Bolsonaro said, according to AFP news agency.
"Maybe there was an accident, maybe they were executed," he was quoted as saying.
The huge region, which borders Peru, is home to around 6,300 indigenous people from more than 20 groups and is threatened by illegal miners, hunters, loggers and coca-growing gangs.
Mr Phillips, who is based in the northeastern city of Salvador, had previously accompanied Mr Pereira to the Javari Valley in 2018 for a story in The Guardian.
Their last known movements
Two indigenous rights groups sounded the alarm about the men's disappearance on Monday.
The men had been travelling by boat in the Javari Valley to interview members of an indigenous guard, the groups said in a statement.
The area - in the west of Amazonas state, near the border with Peru - has seen incursions from illegal loggers and miners.
On Sunday, they stopped in São Rafael where Mr Pereira was scheduled to meet a local leader.
The rights groups said the pair arrived at 06:00 local time and set off shortly afterwards towards Atalaia do Norte, a journey which takes around two hours.
When they failed to arrive, rights group Univaja sent out a search party at around 14:00 but found no trace of the two men along the stretch of river they had been expected to take.
The last to see them were residents of São Gabriel, a community downriver from São Rafael, who spotted their boat going past, the statement said.
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