A Human Rights Watch report on Tuesday found that more than 300 people have been killed over the past decade in conflicts over the use of land and resources in the Amazon, many by organised criminal networks profiting from illegal deforestation.
Of those cases, only 14 were tried in court, the nonprofit said in the report based on 170 interviews.
"This really shows the level of impunity," Cesar Munoz, a senior investigator at Human Rights Watch told Reuters on the sidelines of an event in Sao Paulo to discuss the report. "There is really a failure of investigation and accountability."
The president's office in Brazil did not respond to a request for comment.
Environment Minister Ricardo Salles, responding to the report, told Reuters the government has combated criminality, including in the environmental sphere. He pointed to the mobilizing of troops to combat illegal fires and other environmental crimes in recent weeks.
About 60% of the Amazon rainforest, considered a crucial barrier against climate change, lies in Brazil. Destruction of the forest has surged this year, and the highest number of fires since 2010 has drawn worldwide condemnation of the policies of President Jair Bolsonaro, who advocates opening the Amazon to development.
Human Rights Watch travelled to several Brazilian states between 2017 and the first half of this year to research the report, which showed that almost half of the murders linked to deforestation took place in the Northern state of Para.
The town of Novo Progresso, in Para, recently made headlines for a "day of fire," in which prosecutors suspect a coordinated group set a series of blazes to burn forest and pasture land on Aug. 10.
"In most of the killings that we examined, the victims had received threats or had been attacked before. If the authorities had taken their complaints seriously, these people might be alive today," Daniel Wilkinson, managing director for the Americas at Human Rights Watch, told reporters.
Bolsonaro has weakened Brazil's environmental enforcement agency Ibama, cut its budget by 25% and restricted the ability of field agents to torch the equipment of those found committing environmental crimes, Reuters has reported.
Marina Silva, a former environment minister and presidential candidate, said the report was evidence of Brazil's backsliding on the environment.
"The little that we achieved in the past is now being taken apart," she said.