Congo's government said Wednesday it will investigate the deaths of an American and a Swedish expert for the United Nations and their interpreter, whose bodies were found in a shallow grave Monday after the team disappeared more than two weeks ago.
Sweden said it was opening a murder investigation, and the U.N. Security Council strongly condemned the killings.
American Michael Sharp, Swedish national Zaida Catalan and their interpreter Betu Tshintela went missing March 12 along with driver Isaac Kabuayi and two motorbike drivers in Central Kasai province while looking into recent large-scale violence and alleged human rights violations by the Congolese army and local militia groups.
A U.N. Security Council diplomat, speaking on condition of anonymity because of the sensitivity of the issue, said the experts were investigating alleged mass graves.
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It was the first recorded disappearance of international workers in the once-calm Kasai provinces, where the Kamwina Nsapu militia has been fighting security forces since last year. More than 400 people have been killed and more than 200,000 displaced since government troops killed the militia's leader in August, according to the United Nations.
The U.N. Security Council issued a statement Wednesday night saying its members condemned the killings "in the strongest terms" and also expressed concern at "the unknown status of the four Congolese nationals accompanying them."
Council members called on the Congolese government "to swiftly and fully investigate these crimes and bring the still unidentified perpetrators to justice" and backed a U.N. inquiry, stressing that attacks targeting civilians "may constitute war crimes under international law."
U.N. Secretary-General Antonio Guterres has said the world body would conduct an inquiry into the deaths, saying the cause had not yet been determined. He called on Congo to do the same.
U.N. deputy spokesman Farhan Haq told reporters the U.N. hopes the cause of death of the two experts "will be determined following a more thorough examination."
"Given that they have been found dead, we believe that there's likely grounds that this was a form of criminal act, but we need to determine cause of death," he said.
Haq said the U.N. will look into "what their security conditions had been, what might have happened to them, as well as, of course, to the four Congolese nationals ... for whom we are continuing to search."
He said there were other remains found at the site that still need to be analyzed.
Sharp, from western Pennsylvania, and Catalan were "killed senselessly," the U.S. ambassador to the U.N., Nikki Haley, said in a statement.
Sweden's national police said it has initiated a murder investigation into Catalan's death, and Sweden's prime minister urged Congo to investigate.
Congo government spokesman Lambert Mende said authorities would look into the deaths and seek the U.N. experts' missing Congolese colleagues.
Mende also said Congolese authorities also will look into other recent violence in Central Kasai province, including the deaths of 39 police officers who had been killed by militia forces.
The U.N. last week reported the discovery since January of more than two dozen mass graves in three Kasai provinces. And five videos have emerged in recent weeks that appear to show Congolese soldiers firing on militia members.
While the violence is linked to local power struggles, there are also clear ties to Congo's current political crisis, according to Human Rights Watch.
Anger has been growing in the country at long-delayed presidential elections, and dozens were killed in December amid protests as President Joseph Kabila stayed on past the end of his mandate. A deal reached between the ruling party and opposition to hold elections by the end of this year, without Kabila, remains fragile as the U.N. urges its implementation.
Security forces have been known to back local leaders seen as loyal to Kabila, while militia groups support those who are believed to support the opposition, the rights group said.