Within hours, Valmir Mota de Oliveira, 42, and known as “Keno” would be dead, killed execution-style by two shots to the chest. A security guard was also killed in the shooting.

Keno died trying to stop the development of a research farm for genetically modified soya and corn next to the environmentally sensitive Iguacu National Park, becoming in the process a martyr for the anti-GM movement.

What happened at the seeds research site of the Swiss multinational Syngenta is hotly disputed. What is agreed is that the land invaders – who had been evicted from the same farm in July – set off fireworks as they arrived on the morning of 21 October, causing the unarmed guards to flee and seek help. Within a few hours, an armed militia showed up at the farm on a minibus and, shortly afterwards, Keno was killed and several more protesters were seriously injured. What role Syngenta may have played in ordering the militia to drive away the peasants is at the centre of a bitter dispute. It has turned the incident at its Cascavel research farm into a cause célèbre for the landless workers movement in Brazil where four million peasant families are trying to get access to farmland.

For Syngenta, which was formed from an alliance of Novartis and Astra Zenica, the episode has turned into nightmare of accusation and counter-accusation amid suspicion that it gave free rein to an armed militia to protect its lands as it develops GM corn and maize seed for the expanding Brazilian market.

“Here we have a European company, Syngenta, effectively going around shooting people on its farm,” said Sarah Wilson of Christian Aid which helps fund the Movement of Landless Workers (MST) in Brazil.

Syngenta says it does not know exactly what happened on its farm 10 days ago and that it has sent a team of lawyers from its headquarters in Basle to investigate.

“We don’t know what happened and we are waiting for a full police report,” said a company spokesman, Medard Schoenmaeckers, while strongly denying accusations from the landless farmers that it sent an armed militia to the farm to evict them. “We have a specific clause in our contract with the security firm stating that at no time can the guards carry or use arms,” he said. “Until the police issue a report, I don’t want to speculate about what happened.”

The farmers organisation has issued a detailed description of what it claims happened. “A Via Campesina encampment located at Syngenta’s 127-hectare farm … was attacked by an armed militia. During the brutal attack, a leader and activist … was killed at point-blank range.”

Two other MST leaders were pursued by the gunmen but managed to escape. “We are sure that they came here to kill Keno, Celinha and me,” said Celso Barbosa, one of those who escaped, adding that they had both received death threats since the beginning of the year. Several workers were seriously injured in the clashes.

Amnesty International was quick to express its concern with the apparent use by Syngenta of an “armed militia” which the landless farmers movement says acted through a front company, NF Security, controlled by a rural producers organisation linked to agribusiness.

Threats and intimidation by landowners are common in Parana province, according to Amnesty. As recently as 18 October, local human rights groups presented a dossier of evidence to the state human rights commission complaining about armed men hired by landowners and agricultural companies.

They complained that they often used violent and illegal methods forcibly to evict, threaten and attack activists squatting on land.