As soon as my conversation with Father Mick ended, I grabbed my camera and tape recorder and took a taxi to the airport. The Irish priest, working for a human rights group in the Brazilian city of Manaus, had told me that in a village called Centro dos Aguiar, 2,000km away in the state of Maranhão, a squad of police, accompanied by hired gunmen, had just burned down the houses and expelled the peasants, so that two local ranchers could seize their land. It was the story I had been waiting for.

Forty hours later, on 17 September 1989, I arrived at the friary where the expelled villagers had gathered. Almost everyone I met had suppurating wounds: rope burns, skin lost where they had been dragged along the ground, raw bruises where they had been beaten with rifle butts. But they were more concerned about the three men the police had taken away, who were being tortured in the local station.

I spent the following week interviewing people and gathering evidence. Then I was warned that, without official authorisation to continue, my investigation might jeopardise the survival of the three imprisoned men. I managed to persuade the secretary of security in the state capital, São Luís, to write me a letter of permission.

I was advised by the lawyers representing the peasants to leave the state immediately: the local police might now come for me

My first call was to the police sergeant in the town of Lago dos Rodrigues, who was holding the men. I handed him the letter. He took it from me and put it into a blue folder. I told him the letter was for him to see but mine to keep. He flatly refused to give it back. He then denied everything I had seen with my own eyes. The following day I walked to Centro dos Aguiar, in the hope of interviewing the ranchers.

The village was almost deserted. Most of the houses had either been burned or were boarded up. But on the stoop of one of the larger buildings sat an elderly man. I recognised him from his description: he was the brother of one of the two ranchers, and was said to have been responsible for a number of atrocities.

I took out my tape recorder and camera. He laughed at my questions, telling me, “I like to live in peace.”

As these words left his mouth, three men appeared from behind the house, armed with rifles, revolvers, a machete and a pump-action short-barrelled shotgun. None of them had uniforms. They nodded to the old man and stepped indoors. When they came out, they crossed the road to the big house where one of the ranchers – the man’s brother – lived.

Forest and neighbourhood rub shoulders in Manaus. Photograph: Reuters

They must have reported me, because almost immediately eight military policemen emerged from the ranch house, all armed with rifles except the captain, who had a short shotgun. I stepped forward to speak to them, but with scarcely a word they grabbed me, pinioned my arms, tipped my rucksack upside down and started taking my tapes and films. I remembered with a sick jolt that among them was a tape I’d forgotten to remove from my bag, containing interviews with some of the peasants I’d spoken to. It could jeopardise their lives.

I told the police they had no right: I had a letter of authorisation. “So where is it?” the captain asked me. I explained that the sergeant in Lago dos Rodrigues had taken it. “If you had a letter,” he said, “you could have your films and tapes back. But without the letter you have no right to be here.” I tried to grab my tapes, but I was knocked to the ground. They let me get up, then kicked me down the path.

At dawn the next day, I set off for Lago dos Rodrigues. I had to get the letter then reclaim my tapes before the police in the ranch house played them. My determination rattled the police sergeant, who made a show of looking for my letter – in the wrong folder. I could see the blue one halfway down his stack. I leant across the desk, knocked the heap on to the floor, apologised and returned them all except the blue one, that I opened under the table. I extracted the letter and slipped it into my pocket.

This time, I got a lift to Centro dos Aguiar in a battered pickup. As we parked, I saw the police captain running across the fields towards me, with three of his men. I held out the letter. “Could I have my tapes and films back please?”

He pushed my hand away and his men propelled me towards the car. I resisted, demanding the return of my property. The captain broke his shotgun and inserted some cartridges. I ran, ducked behind the bodywork of the car, and jumped on to the bed of the pickup as it took off.

I was advised by the lawyers representing the peasants to leave the state immediately: the local police might now come for me. The day after I arrived in Manaus, I returned to Father Mick’s office to tell him what I had seen. He was on the phone, talking in English. I waited for him to finish but he passed the phone to me. “You might be able to help with this,” he said.

It was the Guardian’s correspondent at the time, Jan Rocha (this was long before I had a relationship with the newspaper). She told me she was looking into reports in the Brazilian newspapers, suggesting a British journalist might have been killed by police in Maranhão. Did I know anything about it? Apparently he had disappeared in the Bacabal region.

“Really? That’s astonishing. That’s just where I was working. What was his name?”

“I’m not sure how to pronounce it: George, er, Monbiott?”

“Ah, that story might not be entirely accurate.”

I later discovered that a series of escalating rumours had found their way into the national press. I then heard that the state governor, following repeated complaints of torture, illegal imprisonment and murder, culminating in the alleged killing of a British journalist, had been forced to sack the local police and return the stolen land to the peasants. My death had not been in vain.

• George Monbiot is a Guardian columnist