An investigation into the murder of two United Nations (UN) researchers in Democratic Republic of Congo has finished and a trial will soon begin, the army said.

Two suspects out of 16 were arrested after being accused of involvement in the executions of American Michael Sharp and Swedish national Zaida Catalan in Kasai Central province in March. They are charged for war crimes, terrorism and insurrection, military magistrate Colonel Odeon Mwendele Makutu told reporters Saturday in Kinshasa, the capital.

The nation previously blamed a militia known as Kamuina Nsapu for the deaths, without providing a possible motive. The UN and the Swedish and American governments are also conducting inquiries into the killings, and human rights groups say the steps are insufficient.

“Given the implication of Congolese army soldiers in much of the violence in the region in recent months we have serious doubts about the Congolese authorities ability or willingness to carry out a credible independent investigation” Human Rights Watch Central Africa Director Ida Sawyer said by email. “It is critical for the UN to establish an independentinvestigation team.”

The phone of the spokesman for the UN Mission in Congo was turned off when Bloomberg called seeking comment.

Sharp and Catalan, members of a UN panel that monitors sanctions on Congo, went missing on March 12 while conducting research in the centre of the country. A rebellion has left hundreds of people dead since August and forced more than 1 million others to flee their homes.

Video footage obtained by the military and shown to reporters in the capital last month showed the execution of the two foreign nationals by armed men wearing red headbands. Kamuina Nsapu fighters are said to often wear red material around their heads.

The UN has previously accused the Congolese army of human rights abuses in Kasai-Central. Makutu denied any possible state involvement in the killings.