UN experts Zaida Catalan (L) and Michael Sharp (R) were killed in Kasai, DR Congo while investigating mass grave. AFP
Capital punishment is frequently pronounced in murder cases in DRC, but is routinely commuted to life imprisonment since the country declared a moratorium on executions in 2003.
Dozens of people have been on trial for more than four years over a killing that shook diplomats and the aid community, although key questions about the episode remain unanswered.
Michael Sharp, an American, and Zaida Catalan, a Swedish-Chilean, disappeared as they probed violence in the Kasai region after being hired to do so by the United Nations.
They were investigating mass graves linked to a bloody conflict that had flared between the government and a local group.
Their bodies were found in a village on March 28, 2017, 16 days after they went missing. Catalan had been beheaded.
Unrest in the Kasai region had broken out in 2016, triggered by the killing of a local traditional chief, the Kamuina Nsapu, by the security forces.
Around 3,400 people were killed, and tens of thousands of people fled their homes, before the conflict fizzled out in mid-2017.
Prosecutors at the military court in Kananga had demanded the death penalty against 51 of the 54 accused, 22 of whom are fugitives and are being tried in absentia.
The charge sheet ranged from "terrorism" and "murder" to "participation in an insurrectional movement" and "the act of a war crime through mutilation".
According to the official version of events, pro-Kamuina Nsapu militiamen executed the pair on March 12, 2017, the day they went missing.
But in June 2017, a report handed to the UN Security Council described the killings as a "premeditated setup" in which members of state security may have been involved.
During the trial, prosecutors suggested that the militiamen had carried out the murders to take revenge against the United Nations, which the sect accused of failing to prevent attacks against them by the army.
If so, those who purportedly ordered the act were not identified throughout the marathon proceedings.
Among the main accused was a colonel, Jean de Dieu Mambweni, who prosecutors say colluded with the militiamen, providing them with ammunition. He has denied the charges and his lawyers say the trial is a set-up.
Mambweni was among those originally facing the death penalty, but instead was only sentenced to 10 years in jail for "disobeying orders and failure to assist a person in danger". His defence team said he would appeal the verdict.
Two more detainees were acquitted, including a journalist.
Saturday's verdict is liable to appeal at the High Military Court in Kinshasa, DRC's capital.