When Mauro Antony leaves home each morning the Fera leave with him. When he pops out for lunch, the Fera go too. And when he drives to the gym or the courthouse where he works, the Fera, as ever, are there.

The Fera are Manaus's special rescue and assault team – a group of elite police operatives who keep order in this sweltering jungle city. Mauro Antony, a cauliflower-eared, jujitsu fighting 45-year-old, is an Amazon anti-narcotics judge who enjoys their 24-hour protection for one simple reason: in these parts many would prefer him dead.

"We deal with people linked to drug trafficking and organised crime," said the muscle-bound judge with a reputation as a durão – a toughy. "It's a precaution. I've been threatened. After one hearing I was told I would be killed. But it doesn't worry me. I have great faith and I put my work first. You can't be a judge if you are afraid of judging."

Threats to Amazon law enforcement officials and narco-judges are real. One of the judge's cousins was recently targeted with a bomb, apparently a reaction against his relative's firm stance against local mafiosi.

In November, meanwhile, two Brazilian federal agents were killed on the Solimões river, not far from Manaus, after attempting to search a boat travelling at night. Four Peruvian narco-traffickers – allegedly working for the Amazonian drug kingpin Jair Ardela Michue – opened fire on the agents, riddling their boat, and bodies, with high-calibre bullets. It was the first attack of its kind on Brazilian soil.

At least half a dozen Manaus law enforcement officers and prosecutors currently receive round the clock attention from the Fera – black-clad, balaclava-hooded operatives whose logo is a skull skewered on two daggers and a rifle.

Some speak openly about the threats.

Is Thomaz Vasconcellos, Manaus's stocky but affable intelligence secretary, afraid of being assassinated? "Yes."Does he carry a gun? "Always – as we say in the police: we have broken lots of people's credit cards."

Divanilson Cavalcanti, a 40-year-old civil police chief, is equally frank. "The gun never leaves my belt. Not even when I go to the bathroom."

"People I know have had calls saying my days were numbered. They went to my house. We had to move house. We changed our habits exactly so that our family could live in peace," he says, swiftly correcting himself: "Apparent peace."

Others are less forthcoming. Weeks of emails and telephone conversations with one veteran underworld observer, who has also received death threats, resulted in an edgy 30-minute interview, conducted in a half-whisper while stomping around a crowded shopping mall. "I don't go out at night," the interviewee explained, eyes everywhere. "I only go to public places."

Manaus, the capital of Amazonas state, is a major tourist destination for khaki-wearing nature enthusiasts and one of the host cities for the 2014 World Cup. Hemmed in on all sides by thick rainforest, it is located over 2,500 miles (4,000km) north-west of Rio de Janeiro.

Yet for all its isolation Amazonas is umbilically connected to Rio's conflict-ridden slums. Much of the cocaine sold in Rio is said to arrive through Tabatinga, a smuggling mecca lost on Brazil's tri-border with Peru and Colombia, around 700 miles upriver from Manaus.

"Rio state does not produce its own drugs or guns. These are imported from Colombia, Peru and Bolivia," Rio's former security secretary, Marcelo Itagiba, told US diplomats in 2005, according to one WikiLeaks cable. "Representatives from the major drug gangs in these countries operate in Rio," Itagiba claimed.

"We do not have any arms factories or coca fields in Rio de Janeiro," Wilson Carlos Carvalho, a security adviser to Rio's governor, told diplomats this year, according to another leaked cable from the US consulate in Rio. "More needs to be done to control the borders."

Tabatinga's strategic importance to traffickers has turned Manaus, through which the drugs pass on their way south, into a centre for drug-related crime and an HQ for the region's mafia.

"Here the trafficking never stops," said Mauro Antony. Amazonas's federal police had seized at least three tons of pure cocaine this year alone, he pointed out.

"When I started in the force we'd throw a party if we seized 1kg," said Mário César Medeiros Nunes, the bear-like head of Manaus's civil police. "Now we seize 20kg-30kg no problem. Sometimes as much as 200kg or 300kg. The profits are huge. You buy one kilo of cocaine at the border for R$3,000 [£1,160] and sell it here for R$25,000. Nothing else provides such high profits."

Rio's drug conflict has claimed tens of thousands of lives since the 1980s; in Manaus the drug trade is also taking its toll. A growing local market for cocaine has triggered a rise in homicides. Official figures show the number of murders rose over by 9% this year.

"Today 70% of the murders in Manaus are linked to drug trafficking – it is either score-settling, or debt-settling, or problems arising from dependence," said Nunes.

Most of the victims are unemployed men in their mid-20s. Some are not.

The day before the Guardian's interview with Nunes, the corpse of a 10-year-old girl was dumped on the city's outskirts. Her body was swaddled in plastic bags; her hands bound with a piece of washing-line. She wore a bright yellow Brazil football jersey.

Off the record, police suggested Carla Ferreira de Abreu had been murdered because her boyfriend, 14, failed to pay off a drug debt. "I believe it was revenge," said one investigator.

• This article was amended on 7 January 2011. The original said that Tabatinga is around 700 miles downriver from Manaus. This has been corrected.