The court stirred up anger by acquitting the farm-owner who was accused of paying for the killings

A Brazilian court has convicted two men for the murder of prominent Amazon activists, but stirred up anger by acquitting the farm-owner who was accused of paying for the killings.

José Cláudio Ribeiro da Silva and his wife, Maria do Espírito Santo, were shot in May 2011 by gunmen riding on a motorbike through a forest reserve in the northern state of Pará.

Their assassinations in May 2011 generated international condemnation, and were compared by many with the killings of Chico Mendes 25 years ago, and of another environmental campaigner, the American-born nun Dorothy Stang, in 2005.

The couple had been leading the campaign against illegal deforestation and the eviction of rural workers by a local farmer, José Rodrigues Moreira. After multiple threats, da Silva had predicted his own death six months before it happened.

A judge in Marabá in northern Brazil convicted two men for the murders. Lindonjonson Silva Rocha was sentenced to 42 years and eight months and Alberto Lopes do Nascimento got 45 years.

But Moreira was acquitted due to insufficient evidence. Prosecutors claimed Moreira had paid for the assassinations because da Silva and Santo were campaigning against the eviction of three families occupying primary forest he had bought in the Nova Ipixuna reserve with the intention of turning it into cattle pastures.

"The verdict was to a certain point a positive one because those who shot the guns were convicted," Edmundo Rodrigues Costa, the national co-ordinator of the Catholic Land Pastoral watchdog group that tracks land-related violence, told the Associated Press.

Brazil recorded almost half of the reported killings of environmentalists worldwide in 2011, the majority of which were connected to illegal forest clearance by loggers and farmers in the Amazon and other remote areas, often described as a "wild west". Many cases go unsolved.

Last month the Pastoral Land Commission said 32 rural activists were killed in Brazil last year, an increase of 10% from 2011.

Pará is one of the most dangerous areas. Between 1996 and 2010, 231 people were killed and 809 received death threats in this state, according to the commission.

"Violence is the instrument of local capitalism," Brazilian political ecologist Felipe Milanez told reporters this week. "They're proud to kill and they're seen by some as local heroes defending their property with their blood. It's insane, but it's what happening there."

The role of the authorities is mixed. Despite death threats and being named by Brazilian human rights groups as being at high risk of assassination, da Silva was not under police protection. After he was killed president Dilma Rousseff ordered the protection of dozens of other activists and informers who had received death threats. But her administration has also revised the Forest Code to reduce punishments for illegal landowners, and pressed ahead with infrastructure projects that are another cause of tension.

This week the Munduruku indigenous group in Pará criticised the government for using troops to push ahead with a dam survey mission in an area they claim as their territory.