A United Nations inquiry found that two UN investigators were murdered by a group of Congolese fighters, likely members of a local armed group from the central Democratic Republic of Congo, but an absence of evidence "does not preclude the possibility that others are involved".
Michael Sharp, an American who was the coordinator of an independent sanctions monitoring group, and Zaida Catalan, a Swede, were killed in central Congo on March 12 while carrying out investigations for a report to the UN Security Council.
The bodies of Sharp and Catalan were found in a shallow grave two weeks later. UN Secretary-General Antonio Guterres set up an internal board of inquiry and gave an executive summary of the findings to the Security Council on Tuesday.
"A group of Congolese, likely militia members from the Kasai Central province, was responsible," read the inquiry's executive summary, seen by Reuters on Wednesday. "It also found that there was a reasonable likelihood that the killings were committed after consultation with other local tribal actors."
"Without further investigation and the necessary judicial processes, the identity, affiliations, and motives of the group that participated in killing Mr. Sharp and Ms. Catalan cannot be fully established," the report said.
The Congolese government screened a film to reporters in Kinshasa on April 24, which they said showed members of the anti-government Kamuina Nsapu militia killing the UN experts. The pair were shot and Catalan was subsequently beheaded.
More than 3,300 people have been killed and 1.4 million forced to flee their homes in Kasai since the start of an insurrection nearly a year ago by the Kamuina Nsapu armed group, which wants the withdrawal of military forces from the area.
Many analysts say the grainy video of the murders raises more questions than it answers, such as why one of the assassins from the Tshiluba-speaking armed group gave orders in Lingala, which is the language of western Congo and the army.
"It is the judgment of the board that information circulating regarding the possible involvement of various government individuals or organisations does not provide proof of intent or motive," the UN inquiry said.
"An absence of evidence however does not preclude the possibility that others are involved," it said.
A Congolese military prosecutor has said there was no evidence Congolese forces were involved in the murders.
The Board of Inquiry recommended that the Congolese government conduct a criminal investigation with the support of other member states. The inquiry said Congolese authorities had arrested a dozen people and would try them in a military court.
"It's naturally my intention to do everything... with the Congolese government and with the Security Council for the criminals to be punished," Guterres told reporters on Wednesday.
The remaining members of Sharp and Catalan's monitoring group recommended last month that the Security Council ask Guterres to establish an independent international investigation. The United States, Sweden and rights groups have also called for Guterres to establish a special investigation.
"We can't rely on the Democratic Republic of Congo government to find the killers since Congolese security forces may have been responsible for the killings," said Human Rights Watch UN director Louis Charbonneau.
Al Jazeera's Rosiland Jordan, reporting from the UN in New York, said that Guterres is going to present at least three recommendations to the UN Security Council in the coming days on how best to hold those responsible to account.
"The report suggests there needs to be a way to put those [accused] on trial for those murders but it's not clear which option the Security Council may choose to adopt," she said.