Mariano Castro strikes up a chant on the steps of Asunción’s courthouse: “Dictatorship, never again!” It’s 11 July and a controversial trial in the Paraguayan capital has just come to an end. The court ruled that three of his sons participated in a deadly ambush that killed six police officers. Two received lengthy jail terms. The third, according to the verdict, was shot dead as the attack unravelled.

Castro, however, tells a different story. “My son was executed,” he says, over the singing of other protesters. “He had his three-year-old son in his arms when the shooting began. He tried to surrender to the police, but they shot him in the leg, and then in the head.”

The Castro brothers were among 60 small-scale farmers, or campesinos, staging a land occupation near the town of Curuguaty. On 15 June 2012, 350 police officers arrived to evict them. Amid chaotic scenes, 11 farmers and six police were killed. No police were indicted; 11 occupiers were given jail sentences of up to 35 years.

Castro’s account is supported by Codehupy, an umbrella organisation of human rights groups. “There is evidence to suggest that seven of the 11 campesino deaths were the result of extrajudicial executions,” says Oscar Ayala, its executive secretary. “There were serious contradictions in the prosecution’s narrative, and a level of proof that was almost non-existent.” The prosecution opposed the defence’s requests for an autopsy, he claimed. Amnesty International says the state “has not provided a credible explanation as to why they did not investigate these deaths”.

For Codehupy, the deaths continue a history of impunity in Paraguay. The organisation has documented the killings of more than 120 campesinos since the end of the dictatorship in 1989, none of which has been investigated. Among these is the “assassination” of Vidal Vega, a key witness to the events at Curuguaty, who was killed by gunmen six months later.

The UN human rights chief, Zeid Ra’ad Al Hussein, said he was “deeply troubled” by the convictions.

The state dismisses claims of irregularity in the Curuguaty trial. “What matters is not what these biased groups say, but what was shown before a sentencing tribunal by myself and my colleagues,” says state prosecutor Liliana Alcaraz.

Alcaraz admits that it was not possible to draw a direct link between the firearms held by the campesinos and the bullets that killed the police. But she denies that this is significant: “Our system says that you can prove a fact through various methods …

“… We trusted the eye-witness accounts of the police because we did a series of scientific tests which proved that the position of the shooter, the position of the victim, the geographic layout, the radiography, the ballistic analysis – everything that indicated the trajectory of the bullet – supported them.”

When asked about the lack of an autopsy, she replied: “Effectively, there was an autopsy – an autopsy is a procedure established in the legal code which has the effect of determining the cause of deaths, and in this case the causes of the deaths of the police victims were firearms, the majority shotguns.”

There is a troubling precedent of legal interference in Paraguay. In 2011, Gustavo Bonzi, a magistrate in the department of Concepción, saw his career end after acquitting a group of 12 campesinos. “I’d worked for five years without any problems,” says Bonzi. “But in this case, the prosecution presented their argument in a totally generic way – the accused helped guerrillas with a kidnapping, so 30 years in jail. But they didn’t even specify what each one was supposed to have done.”

On voicing his scepticism, Bonzi says he was threatened by people involved in the case. Shocked, he rang one of the nine judges who make up Paraguay’s supreme court. “I told him, ‘This is a catastrophe – I’m going to have to acquit these people.’ And he replied, ‘Just do what they tell you. This is a dangerous case.’”

But Bonzi refused to back down. He acquitted all 12 defendants. An appeal tribunal was asked to review it. But before the proceedings got under way, he was stripped of his magistrate’s licence. “They destroyed my future, what I’d spent my life working towards,” says Bonzi. “It was a direct message to the other judges: ‘Confirm what he says, and we’ll throw you out.’”

The tribunal annulled Bonzi’s decision. The case was reopened. Some defendants negotiated reduced jail terms; others fled to Brazil.

“It was a horrible moment,” Bonzi recalls. “I knew the Paraguayan justice system had problems. But I had faith that it was improving. I discovered the reverse. Nobody dares tell the truth. It’s the same with the Curuguaty case – I knew they’d find a way to condemn the campesinos.”

According to the Service for Peace and Justice Paraguay (Serpaj), violence has increased following the victory of Horacio Cartes and his Colorado party in the 2013 elections. “On 15 August Cartes assumed the presidency; on 24 August, military forces were installed in the north,” says Cristina Coronel, a researcher with Serpaj.

These forces were part of a specialised unit, the Fuerza de Tarea Conjunta (combined taskforce, or FTC), drawing on military and police resources. The stated aim was to neutralise a guerrilla group, the EPP. But questions hang over taskforce’s priorities.

“They operate in an area with rich water resources and huge demand for land for soya, cattle and marijuana,” says Coronel. “It’s also where campesino movements have historically been most organised and put up the fiercest resistance.”

In 2015, Serpaj published an investigation into FTC abuses. It describes the militarisation of the northern department of Concepción, studded with checkpoints and patrolled by soldiers, tanks and helicopters. It documents accounts of beatings, killings, death threats and torture. And behind all this, it traces a justice system defined by impunity and arbitrariness.

Leonidas Villalba, a mother of 10 living on a smallholding, has experienced this first-hand. In December 2015, her husband, Julian Ojeda, went out to fish in a nearby river. A couple of hours later, Leonidas heard shots. Julian didn’t return. The next day, a neighbour told her he’d been killed by the FTC; she’d seen it on the lunchtime news.

“They say he was a member of the EPP but I don’t know why they thought that. No one has come here to explain anything,” Villalba says. “Nobody visits me now. Everyone is afraid of being associated with Julian. I feel totally abandoned.”

Four years after the 17 deaths in Curuguaty, tensions over land have intensified.

“We’re reaching a point where most Paraguayans have lost their trust in the justice system,” concludes Codehupy’s Ayala. “This undermines our whole democracy.”

Paraguay’s ministry of the interior declined to be interviewed for this article.