UNITED NATIONS (Reuters) - Several members of the U.N. Security Council called on U.N. Secretary General Antonio Guterres on Thursday to establish an international investigation into the murders of two U.N. investigators in Democratic Republic of Congo earlier this year.

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Michael Sharp, an American who was coordinator of a sanctions monitoring group that reports to the council, and Zaida Catalan, a Swede, were killed on March 12 while carrying out investigations in central Congo’s insurrection-ravaged Kasai region.

An internal board of inquiry said in a report this week that the assassins were likely militia members from the area. In a separate report this month, discussed by the Security Council on Thursday, Sharp and Catalan’s colleagues said they could not preclude the involvement of state security services.

At the council meeting, the United States, Britain, France, Sweden and Japan urged Guterres to establish a follow-up investigation to determine responsibility. The United States has called for such an inquiry since June.

“We reiterate ... our call for the secretary general to immediately put in place ... a special mechanism of investigation to allow the competent judicial authorities to carry out effective proceedings so that justice is rendered,” said France’s deputy permanent representative, Anne Gueguen.

Congolese authorities have arrested nine suspects in the killings but some Western governments and rights groups are skeptical that the real masterminds have been identified.

In a letter to the president of the Security Council this week, Guterres said he planned to discuss the establishment of a “follow-on mechanism” to the board of inquiry with Congo officials and council members.

“Accountability has yet to materialize,” said Jonathan Allen, Britain’s deputy permanent representative. “Those who ordered their killings remain at large.”

The violence in Kasai has killed more than 3,300 people since the start of an insurrection nearly a year ago by the Kamuina Nsapu militia, which wants the withdrawal of military forces from the area. The United Nations has repeatedly accused Congolese forces of using excessive force.

Congo’s government denies its troops have engaged in systemic abuses and reiterated its opposition to an international probe on Thursday, with Foreign Minister Leonard She Okitundu telling the Security Council that authorities are already cooperating with American and Swedish investigators.

It received support from Russian ambassador Vassily Nebenzia, who told the council: “We would like to warn about attempts to put all the blame about what is taking place on the Congo army and police without any evidence.”