UNITED NATIONS (AP) — The United States called on Secretary-General Antonio Guterres on Monday to initiate a special investigation into the gruesome March killings in Congo of two United Nations experts from the U.S. and Sweden and their interpreter.

The U.N. is conducting a board of inquiry into the deaths of American Michael Sharp and Zaida Catalan of Sweden. But U.S. Ambassador Nikki Haley the Trump administration wants a special investigation – which would be a criminal investigation.

Haley said in a statement that the United States will also support a resolution in the Human Rights Council in Geneva to establish a commission of inquiry into human rights violations in Congo’s central Kasai region, where the killings took place. The Trump administration is considering pulling out of the council and she is scheduled to address its members Tuesday.

Haley said Sharp, Catalan and their families “deserve justice.”

“We owe it to their legacy to end the human rights abuses being carried out by armed groups and (Congo’s) government against the Congolese people,” she said. “We hope other nations will join us in supporting a U.N.-led investigation and Human Rights Council-led inquiry into these horrific acts.”

U.N. spokesman Stephane Dujarric said the United Nations has repeatedly called on the Congolese government “to conduct a complete and thorough investigation into the murder of our colleagues and the fate of the four Congolese men who accompanied them.” He said those responsible for the murders must be brought to justice and stressed that the government “has the duty and responsibility to carry out a criminal investigation.”

But Dujarric stressed that the United Nations Secretariat “cannot substitute (for) the criminal justice system of a sovereign country.”

The U.N. board of inquiry is expected to issue a report and recommendations by the end of July, he said. “We stand ready to support any investigative mechanism set up by the relevant inter-governmental bodies,” he added.

The two U.N. experts and interpreter Betu Tshintela went missing while investigating human rights abuses in Congo. Their bodies were found in a shallow grave.

The Congolese government later obtained a cellphone video showing them being killed. The government blamed members of the Kamwina Nsapu militia who are active in Kasai, where at least 400 people have died since August.

Congolese authorities have launched an investigation and said late last month they have 16 suspects in connection with the murders.

Six suspects appeared in a Congo military court in Kananga, the provincial capital of Kasai Central, on Monday to answer charges. Two are accused of murder and four of facilitating the escape of suspects.

Congolese Attorney General Flory Kabange Numbi said last month that authorities have also opened an investigation into whether former government minister Clement Kanku played a role in inciting violence in the Kasai provinces. Kanku was one of the people being investigated by the U.N. team that was killed.

The announcement came after The New York Times cited an audiotape of Kanku allegedly condoning the violence, at one point saying: “It’s good that we burn everything.”