Sairá Ka’apor patrolled one of the most murderous frontiers in the world, a remote and largely lawless region of the Brazilian Amazon where his indigenous community has fought for generations to protect their forest land.

Armed with clubs, bows and arrows, GPS trackers and crude guns, he and fellow members of Ka’apor Forest Guard drove off – and sometimes attacked – loggers who intruded into their territory, the 530,000-hectare Alto Turiaçu Indigenous Land, which is roughly three times the area of Greater London and contains about half of the Amazon forest left in Brazil’s northern Maranhão state. That vigilante role came to an end last April when Sairá was stabbed to death in Betel, a logging town close to Ka’apor territory.

This was a murder that took place in a fragile, dangerous world, balanced precariously between values of conservation and consumption, tradition and modernity. The death has gone uninvestigated by police and unreported by the Brazilian media, but it highlights the violent pressures driving forest clearance.

For decades, loggers have cut dirt tracks into the forest that allow them to selectively fell valuable timber such as ipê (Brazilian walnut), which can fetch almost £1,000 per cubic metre after processing and export. This is followed by fires – often set deliberately – that destroy the remaining trees so land can be used for cattle ranching or soy farming.

Last year 6,624 sq km – more than four times the area of London – was deforested in Brazil. This was the first time in three years that the rate did not rise, and the country remains off track to reach its Paris climate targets. Numerous studies have shown that protection of indigenous land is the most effective way to cut deforestation, but the Ka’apor – like many other tribes – feel the police often work against them. Battling to save the forest is a risky business. According to Global Witness, Brazil is the deadliest country in the world for environmental and land defenders with 44 killings recorded in 2017. Maranhão – the nation’s poorest state – is among the worst affected. There were more death threats and attacks on indigenous groups here than anywhere else in 2016, according to the Pastoral Land Commission.

Sairá knew the dangers.

“He was utterly fearless,” recalls a senior member of the Ka’apor council, Itahu, who described how his fellow defender was in the vanguard of a successful operation in 2014 to intercept and burn three logging trucks.

The village of Ximborenda is home to the 2,200-strong Ka’apor community where Sairá lived. At night, fireflies glow while frogs and insects provide an undulating chorus of noise. Tarantulas crawl along bedroom walls. The biodiversity is testimony to the quality of the forest in a place that defiantly holds out against extractive industries and global markets. But the pressure on this natural wealth is relentless. The Ka’apor council has attempted to hold the line but many individuals succumb to temptations.

Hira, a Ka’apor indigenous leader and forest guardian, finds an illegal logging truck. Photograph: Lunae Parracho/The Observer

Nothing is black and white. The loggers were also neighbours. Many were poor. There was intermarriage. People would fight the loggers one year, befriend them the next then go back to fighting. Even Sairá once sold trees on his land in exchange for cachaça and cash, but later gave up alcohol and led the campaign to stop others in his village from drinking and trading wood.

“The loggers use alcohol to weaken us. It’s a more powerful weapon than guns,” said Itahu. He believes Saira was murdered as part of a long-running intimidation campaign. Many councillors have experienced threats. Sarapo Ka’apor described how he was captured by a gang of loggers in 2013. “They held my arms behind my back and fired their guns repeatedly so close to my ears that I went temporarily deaf. One bullet grazed my scalp. I was drenched in blood.”

On the road to Betel the next day, it was clear where the real power lies. More than any state in Brazil, Maranhão is in thrall to the “coronels” (major landowners who carry a semi-feudal authority). One family – the Sarneys – have dominated politics here for as long as anyone can remember. The patriarch (an 87-year-old senator who ruled Brazil from 1985 to 1990) has a roadside school named after him – the Escola Unidade da President José Sarney. The system of patronage and control is replicated at a municipal level. The powerbroker near the Ka’apor’s land is Josimar Rodrigues, a state assemblyman who has been accused by police of running an illegal operation to remove timber from the indigenous territory. Despite the allegations, he and his family remain hugely influential. His wife, sister and former driver are all mayors in municipalities that overlap Ka’apor territory.

It wasn’t me who killed Sairá, I didn’t even have a knife. It must have been a branco [white person] who killed him Pracidio Ka'apor

Betel is a poor logging village of a few hundred homes, a church and a bar. It is in the latter that the fight allegedly started that led to Saira’s death. Locals say a group of four or five Ka’apor had been drinking for several hours until Sairá turned up, scolded them and ordered them to return to their village.

Nobody saw the murder. The bar owner’s wife – who gave only the name Donna Raimundo – assumed there had been a fight on the way back. She said Sairá’s sister-in-law came back to the bar, saying he had been stabbed by a relative called Pracidio. But Ka’apor councillors refuse to accept one of their community would kill another. The last time that happened was more than 20 years ago, and the traditional punishment is being buried alive. And Saira and Pracidio had been close friends from childhood. “They went hunting together. I can’t believe he killed him,” said Iratui Ka’apor, who taught them both.

Most people’s suspicions are directed at the loggers in Betel. Sairá had been getting death threats. A month earlier he had requested help against a local businessman who had reportedly been plying the Ka’apor with alcohol so they would agree to sell wood.

In fear of his life, Pracidio had gone into hiding. The Observer tracked him, first by motorbike along the muddy dirt track to Tauaxi Renda – a Ka’apor outpost near the Rio da Sangue (River of Blood) and the notoriously murderous region around Nova Conquista – and then, half a day later, to a neighbouring town, where he was waiting for a bus. A short, wiry man, Pracidio was reluctant to make eye contact. “It wasn’t me who killed Sairá,” he said slowly. “I didn’t even have a knife. It must have been a branco [white person] who killed him.”

He says four illegal loggers and fishermen had previously threatened him with knives and told him to deliver Sairá to them, which he refused. “I didn’t even see him that night. There was no fight at the bar when I was there. I returned to my village on a mule with my son around 9pm. My father saw me.”

Almost certainly, Sairá’s killing will go unsolved. The nearest police station, in Ze Doca, has yet to open a case. Outside the indigenous community, nobody cares. With no clamour for justice, the trail will go cold and all that will have been proved is the banality of murder on the frontline of forest protection.