Campaign group Save the Congo! has urged the United Nations (UN) to order an investigation into the killing of two UN officials and their Congolese translator in the Democratic Republic of Congo's restive Kasaï-Central province.

The Congolese government and later the UN, on 28 March, confirmed two bodies discovered in a shallow grave were those of US citizen Michael Sharp, 34, and Swedish national Zaida Catalan, 36, who went missing on 12 March near Moyo River on the way to the capital Tshimbulu.

The body of their Congolese interpreter, Betu Tshintela, was discovered in the same location. Their three Congolese colleagues, driver Isaac Kabuayi, and two unidentified motorbike drivers are still missing.

The two experts, along with four Congolese colleagues, were travelling in Kasaï after they were instructed by the UN Security Council to investigate reports of human rights violations committed by Congolese security forces against civilians in the south-central province, and the recent discovery of mass graves.

Kasaï, in the heart of the DRC, has been plagued by violence between the Congolese security forces and a local militia, Kamuina Nsapu, who are seeking to avenge the death of their leader, Kamuina Nsapu, who was killed in Tshimbulu by Congo's security forces in August 2016.

In a statement, Congolese-led campaign group Save the Congo said it "condemns in the strongest possible terms the killings".

"The two UN officials were likely targeted because those who ordered the summary executions of civilians in Kasaï and those responsible for the mass graves they were investigating feared their investigation could identify them and evidence from the investigation could potentially be used in future criminal prosecutions," founder Vava Tampa, said on 29 March.

Zaida Catalan and Michael Sharp
United Nations investigators Zaida Catalan (L) and Michael Sharp (R) were kidnapped by unidentified men as they investigated reports of human rights violation in Kasai Central province, DRC Human Rights Watch

The UN investigators' abduction came weeks after the release of several videos appearing to show members of the armed forces targeting unarmed civilians, including children, in the region.

"Mr Sharp and Ms Catalan were abducted and killed carrying out a task the UN Security Council (UNSC) had mandated them to do on behalf of humanity and for the sake of international justice. The UNSC must now make all efforts to identify those who ordered their killing as well as the summary executions of civilians they went to investigate," said Tampa.

Home to late political opposition heavyweight Etienne Tshisekedi, the diamond rich Kasaï-Central province has been one of the most anti–Kabila provinces in the DRC. At the time of the UN team's abduction, and discovery of three of the bodies, Tshimbulu as well as the surrounding region had been under control of the security forces since January, Tampa claimed.

The Kinshasa government said earlier this month the two UN officials had fallen into the hands of unidentified "negative forces" along with four Congolese who were with them.

UN Secretary General, Antonio Guterres, on 29 March vowed to press for justice in the killing of the two experts and their interpreter.

"I trust that the Congolese authorities will conduct a full investigation into this incident. The United Nations will also conduct an inquiry. In case of criminal acts, the United Nations will do everything possible to ensure that justice is done," he said in a statement.