Available evidence in the murders of two U.N. experts in Congo leaves open the possibility that members of the government's security services, a militia operating in the area, or other armed groups were responsible, their colleagues on an expert panel said in a new report.

The report to the U.N. Security Council, seen by The Associated Press on Wednesday, said Michael Sharp of the United States and Zaida Catalan of Sweden "were assassinated in a premeditated set-up" on March 12 in central Congo's violence-torn Kasai region under circumstances that are still unclear.

The expert panel said they retrieved an audiotape dated March 11 in which Sharp and Catalan discussed a field visit the following day with representatives of "the Kamwina Nsapu family" — a militia active in Kasai whose customary chief Jean-Pierre Mponde was killed by Congolese army troops in August 2016.

The panel said "the tape confirmed that the investigation aimed at better understanding Kamwina Nsapu's structure, its support networks and the potential recruitment and use of children."

On March 12, Sharp, the panel's coordinator and expert on armed groups, and Catalan, a humanitarian expert, embarked on the field visit from Kananga, the provincial capital of Kasai Central, toward the locality of Bunkonde, the report said.

The panel said they understand "that around 4 p.m. local time Michael Sharp and Zaida Catalan were executed by a heteroclite group of individuals not yet identified." Heteroclite refers to something "irregular."

The bodies of Sharp and Catalan were found in a shallow grave two weeks later. The Congolese government obtained a cellphone video showing them being killed and blamed members of the Kamwina Nsapu militia.

The experts said members conducted a preliminary analysis of phone records and a video of the murder as well as interviews.

"The preliminary evidence does not yet allow the group to attribute responsibility for the murder," the experts said.

"However, the available evidence does not preclude the involvement of different actors such as (pro- or anti-government) Kamwina Nsapu factions, other armed groups as well as members of state security services," they said.

The report said the Kamwina Nsapu reacted violently to the Congolese army's killing of Mponde, their chief, including by launching attacks against "state agents and state symbols." This prompted the Congolese army and police to launch offensives against the insurgent movement, "often using disproportionate violence," it said.

Dozens of mass graves have been discovered and video footage shows deliberate killings including of unarmed Kamwina Nsapu members, the report said.

"Throughout 2017, this conflict moved away from an insurrection of a specific community towards a larger upheaval, spreading the Kamwina Nsapu label far beyond its initial confines," the experts said, adding that violence has now engulfed vast parts of Kasai, Kasai Central, Kasai Oriental, and Haut-Lomami provinces.

The United Nations is holding a board of inquiry into the murder of the two experts and its report is due by the end of July. The United States and Sweden are conducting separate investigations.

The panel of experts recommended that the U.N. Security Council authorize Secretary-General Antonio Guterres "to establish an independent international investigation" into the murders in order "to gather evidence to identify the perpetrators, their support networks and motives."

U.S. Ambassador Nikki Haley has also called on Guterres to initiate a special investigation into the killings.