Three suspected killers of a couple who blew the whistle on illegal logging in the Brazilian Amazon went on trial Wednesday.

Antonio Filho, a member of Brazil's Catholic Church-affiliated Pastoral Land Commission (CPT) who is monitoring the trial before a court in the Amazonian town of Maraba, said it was expected to last until Thursday .

In the dock are Jose Rodrigues Moreira, who allegedly masterminded the May 2011 ambush killing of Jose Claudio da Silva and his wife Maria do Espirito Santo near Maraba, and the two alleged perpetrators Lindonjonson Silva Rocha and Alberto Lopes do Nascimento.

The three were arrested in a jungle hideout 300 kilometers (180 miles) from Maraba in the northern state of Para.

The environmentalist couple had for years campaigned against loggers and ranchers who force slave labor to clear-cut large swaths of the Amazon.

The murder was the first in a series of 10 over a three-month period in the Amazon, most of them in Para, one of the Brazilian states hardest hit by violence by local loggers and ranchers against rural workers and their supporters.

A report by the CPT said big landowners in the area often enjoy "total impunity".

At the time of the killing of da Silva and his wife, Amnesty International had called on Brazilian authorities to end these killings as well as "the impunity enjoyed by those who incite this violence".

Several Amnesty representatives were due in Maraba Wednesday to monitor the trial.

In Para alone, 914 rural workers, lawyers and church officials were killed over land feuds between 1964 and 2010. Trials were held in only 18 cases and 24 people were convicted.

To date, only six of those convicted are still in jail, including three for the 2005 murder of US-born Dorothy Stang, a 74-year-old nun who defended the rainforest and local landless people persecuted by illegal loggers and landowners in Anapu, according to the CPT report.

The NGO also said 12 of the 29 murders of rural activists in Brazil in 2011 occurred in Para.