Updated at 1:45 p.m. ET March 29

The United Nations has confirmed that two of its employees, who were looking into violence and human rights abuses in the Democratic Republic of the Congo, have been killed.

An American, Michael Sharp, and a Swede, Zaida Catalan, had gone missing earlier this month while traveling in the region. Tuesday, Congolese officials said their bodies, along with that of their interpreter, Betu Tshintela, had been found in Central Kasai province.

Sharp's father, John, had posted to his Facebook page the news that remains had been found that appeared to be those of his son and his son's colleague. "This is a message I hoped never to write," John Sharp said. "All other words fail me."

In confirming the deaths of Sharp and Catalan, U.N. Secretary-General Antonio Guterres said he was deeply saddened:

"Michael and Zaida lost their lives seeking to understand the causes of conflict and insecurity in the DRC in order to help bring peace to the country and its people. We will honor their memory by continuing to support the invaluable work of the Group of Experts and the whole UN family in the DRC."

U.S. Ambassador to the U.N. Nikki Haley said Sharp "selflessly put himself in harm's way to try to make a difference in the lives of the Congolese people. His courage and desire to serve others is an example for us all."

"Both of them did very brave, difficult work in dangerous situations, and it was all in order to expose human rights violations and put pressure on authorities who might be able to do something about it," says Holly Dranginis of the activist group called the Enough project. A friend of Sharp's, she is calling for an independent investigation.

Two years ago, NPR profiled Sharp, who was in his 30s and worked at the time with the Congolese Protestant Council of Churches in their Peace and Reconciliation Program. Correspondent Gregory Warner reported on meeting Sharp on a commuter boat crossing Lake Kivu, and learning about his work.

"Every few weeks Sharp and his church colleagues would walk, unarmed, to the base of the rebels known as the FDLR (Forces Démocratiques de Liberation du Rwanda). There they'd sit in the shade of banana trees to drink tea with the rebels and listen to their stories. Certain sensitive subjects — sexual violence against villagers, recruitment of child soldiers — were no-go, he says, because the aim was building rapport with the rebel faction. 'The more we interact [with them] the more they trust us to turn themselves in to us.'"

Greg quoted Sharp as saying the church program helped persuade some 1,600 fighters to lay down their weapons and leave the forest they'd occupied for two decades.

Describing his work, Sharp had said, "You can always listen. You can always listen to people who want a chance to talk about how they see the world."

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