Two Rwandan police officers have been jailed for 20 years each for the murder of anti-corruption activist Gustave Makonene.
Makonene, who worked for the NGO Transparency International, was killed in July 2013 close to Rwanda's border with the Democratic Republic of Congo.
He had been investigating mineral smuggling from DR Congo to Rwanda and, said prosecutors was murdered after be became aware of the police officers' involvement.
Transparency International said the 20-year sentences were too light.
"We followed the trial carefully and the convicts had been saying that the murder was a planned one. We expected that they would get the heaviest punishment in the Rwanda penal code," the group's Rwanda director, Apollinaire Mupiganyi, told Reuters news agency.
Eastern DR Congo is rich in minerals and has been the subject of years of conflict as rival militias vie for control of the trade.
Smuggling has thrived in the region for years and a United Nations panel in 2012 said funding from the trade had been used to finance the Congolese M23 rebel group. The group was defeated in November 2013.
Kigali has consistently denied funding the group and said it pursues and prosecutes individuals and groups involved in smuggling.