Two Rwandan police officers have been jailed for 20 years for killing an anti-corruption activist who was investigating mineral smuggling from Democratic Republic of Congo.

Gustave Makonene, who worked for anti-graft body Transparency International, was found dead in July 2013 in Rubavu, near the border with eastern Congo, a war-torn region rich in minerals that have fueled an illegal trade.

The two men convicted by the court on Thursday were both police corporals, Isaac Ndabarinze and Nelson Iyakaremye.

Judge Esron Gashyende said they had been sentenced to 20 years rather than life -- the usual term for the crime -- because they had confessed and cooperated with the investigation.

The executive director for Transparency International in Rwanda said the sentence was too light.

"We followed the trial carefully and the convicts had been saying that the murder was a planned one," Mupiganyi Appolinaire told Reuters. "We expected that they would get the heaviest punishment in the Rwanda penal code."

He said he would consider an appeal after examining the full ruling.

Eastern Congo is rich in minerals such as tin, tungsten and tantalum but the Kinshasa government has little control over the huge region that has been fought over by militias and rebels for decades. Smuggling has thrived.

A report by a U.N. panel in 2012 accused Rwandan traders of using the mineral trade profits to fund M23, a now defeated rebel group which experts and Western governments had said was backed by Rwanda.

Kigali denied accusations it had supported the M23. Rwanda also says it vigorously pursues and prosecutes those involved in smuggling or corruption.