Brazil's President Luiz Inacio Lula da Silva has pledged to end a culture of "impunity" surrounding land disputes in the Amazon.
Dorothy Stang had spent decades defending Para's peasant settlers
The president's vow came after police in the Amazon state of Para arrested a third suspect in the murder of US-born missionary Dorothy Stang.
He promised that all those responsible for the killing would be captured.
Lula blamed the violence on reactionary forces opposed to his efforts to preserve the Amazon.
"The government will demonstrate that there will be no impunity, that the Amazon belongs to all of us and that we will protect this territory, without wavering," he said in his regular radio address to the nation.
Sister Dorothy, 74, had campaigned for more than 30 years on behalf of peasant farmers in the Amazon.
She was murdered on 12 February in what police believe was a contract killing.
The third suspect in the case, Uliquelano de Souza Pinto, was arrested by police on Monday in the town of Belo Monte in western Para. Police found him after a tip-off.
Police accuse him of being one of two gunmen who shot Sister Dorothy at close range.
Another alleged gunman, Rayfran das Neves Sales, is already in custody, as is Amair Freijoli da Cunha, who faces conspiracy charges and is accused of hiring the other two to commit the murder.
Police have so far failed to capture a fourth suspect, a local landowner, whom they accuse of ordering the killing. He has denied any involvement in the crime.
Brazil's government is under pressure to bring the perpetrators to justice as the case continues to resonate across the country and beyond, the BBC's Steve Kingstone in Sao Paulo says.
The crime has highlighted the often violent conflicts over land in the Amazon, our correspondent adds.
After the murder, the government said it planned to protect a huge swathe of the Amazon.
It said nearly four million hectares (10 million acres) in Para state would become a conservation area in a bid to ward off loggers and landowners.
The government said it also wanted to reinforce the environmental police force.
Sister Dorothy, who was a naturalised Brazilian, had complained that the government was not doing enough to stop land-related violence.