The campaign group, Human Rights Watch, (HRW) has called for an international investigation into the killing of 17 aid workers Sri Lanka three years ago.
A government inquiry after the killings failed to identify the killers.
The group accused the government of grossly mishandling the investigation into the deaths of local employees of the Action Against Hunger group.
HRW said that an international inquiry was needed into the murders. Sixteen of the victims were ethnic Tamils.
At the time of the killings, European truce monitors said that they believed troops were involved, but the government's own inquiry said the military was not responsible.
The military was engaged in heavy fighting with Tamil Tiger rebels in the region at the time.
"For three years since the massacre, the Rajapaksa government has put on an elaborate song and dance to bedazzle the international community into believing justice is being done," said James Ross, legal and policy director at Human Rights Watch.
The aid workers' deaths were condemned around the world
"It's time the UN and concerned governments say 'the show is over' and put into place a serious international inquiry.
"Instead of doing all it can to get justice for this horrific crime, the Sri Lankan government is further traumatising the victims' families by trying to shift the blame to others."
A presidential commission of inquiry last month publicly announced its findings in the case, which exonerated the Sri Lankan army and navy for the killings.
But HRW says that this report was based primarily on "limited witness testimony" from people who said that the armed forces were not in the vicinity at the time.
The commission blamed the killings on either the Tamil Tigers or auxiliary police known as home guards. Its full report to President Mahinda Rajapaksa remains unpublished.