UN experts Zaida Catalan (L) and Michael Sharp (R) were killed in Kasai, DR Congo, while investigating mass graves. (AFP)

In a mass trial over the 2017 murder of two UN experts in a troubled central region, a military court in the Democratic Republic of the Congo sentenced 51 people to death, including several in absentia.

In the Democratic Republic of the Congo, capital punishment is frequently imposed in murder cases, but it has been routinely commuted to life imprisonment since the country declared a moratorium on executions in 2003.

For more than four years, dozens of people have been on trial in connection with a killing that shocked diplomats and the aid community, but key questions about the incident remain unanswered.

Michael Sharp, an American, and Zaida Catalan, a Swedish-Chilean, vanished while investigating violence in the Kasai region for the United Nations.

They were looking into mass graves connected to a bloody conflict between the government and a local group.

On March 28, 2017, their bodies were discovered in a village, 16 days after they went missing. Catalan had been executed by beheading.

The conflict in the Kasai region had broken out in 2016, caused by the killing of a local traditional chief, the Kamuina Nsapu, by the security forces.

It is worth mentioning that around 3,400 people were killed, and tens of thousands of people fled their homes before the conflict ended in mid-2017.

Prosecutors at the military court in Kananga demanded the death penalty against 51 of the 54 accused, 22 of whom are being tried in absentia.

The charge list ranged from "terrorism" and "murder" to "participation in an insurrectional movement" and "the act of a war crime through mutilation."

Pro-Kamuina Nsapu militiamen executed the pair on March 12, 2017, the day they went missing, according to the official version of events.

Then in June 2017, a report handed to the UN Security Council defined the killings as a "premeditated setup" in which members of state security may have been involved.

During the trial, prosecutors believed that the militiamen had followed through the murders to take revenge against the United Nations, which the group accused of failing to stop attacks against them by the Army.

Among the main accused was a colonel, Jean de Dieu Mambweni, who prosecutors say conspired with the militiamen, supplying them with ammunition. He denied the accusations and his lawyers stressed that the trial is a set-up.

Mambweni was among those facing the death penalty, but he was instead sentenced to ten years in prison for "disobeying orders and failing to assist a person in danger." His legal team stated that he would appeal the verdict.

Two more detainees, including a journalist, were acquitted.

The verdict from Saturday is subject to appeal at the High Military Court in Kinshasa, the DRC's capital.