More than 10 months after a woman was shot dead in the Amazon, two men will go on trial today for the murder of the rainforest activist and American nun, Sister Dorothy Stang.

But as lawyers prepare to lay out the case against the two so-called pistoleros who allegedly shot her, there have been claims that the government is ignoring a wider conspiracy directly related to the destruction of the rainforest.

Rayfran Sales and Clodoaldo Batista, the two men accused of the shooting of the 73-year-old on 12 February in Anapu, a remote rainforest encampment, will go on trial in the north-eastern Brazilian city of Belem. Three other men said to have paid the hitmen to assassinate Ms Stang will go on trial at a later date. But US senators and a UN envoy have said they believe that wealthy loggers and ranchers suspected of actually ordering the killing have escaped justice.

Hina Jilani, a UN special rapporteur on human rights who is attending the trial, said this week: "We look forward to the government taking measures to catch the perpetrators. I have communicated my concern to the government and urged it to ensure impunity does not prevail."

In a letter to the Brazilian President, Luiz Incio "Lula" da Silva, more than 20 US Senators have urged him to press the authorities in Para state to investigate others who may have been involved in the killing. They referred to a recent Amnesty International report which concluded that those behind most of the killings in the state escaped with "impunity". This year, at least 28 rural workers and activists have been murdered in Brazil - more than half in Para state.

Sister Dorothy received numerous death threats during the 30 years she spent in the rainforest of Brazil trying to help landless peasants and small-scale subsistence farmers. She had the facts on her side: since 1970, 20 per cent has been cut down as a result of logging and development - equivalent to eight football pitches a minute.

The murder of the white-haired nun known as the Rainforest Martyr recalled the 1988 killing of the activist Chico Mendes. Like his death, her killing has been seized on by campaigners around the world as underlining the bitter battles being fought in Brazil over control of the rainforest and its future.

One of Sister Dorothy"s brothers, David, who is attending the trial, said: "What we hope is that this is the beginning. It"s a considerable step for the state of Para, which has never held such a trial. I hear the prosecutor is excellent."

Sister Elizabeth Bowyer, the senior sister of the Ohio-based order of the Sisters of Notre Dame de Namur, of which Sister Dorothy was a member, said: "It"s very important that justice is done. It"s very important that murders with impunity stop. We think the trial is very important."

Emily Goldman, of the Washington-based human rights group RFK Memorial, said it was believed that up to 30 people may have been involved in the plot behind Sister Dorothy"s killing. She said the federal government did not have the will to take on the powerful interests in Para state and elsewhere in the Amazon. "This is a historic problem," she said.

The Brazilian Justice Minister, Marcio Thomaz Bastos, has vowed to continue the investigation into Sister Dorothy"s killing in order to pursue others who may have been involved.