The father of one of the United Nations experts killed in the Democratic Republic of Congo (DRC)'s restive Kasai Central province has paid tribute to his late son, 34-year old Michael Sharp.

The Congolese government and later the UN, on 28 March, confirmed two bodies discovered in a shallow grave were those of Sharp, a US citizen and Swedish national Zaida Catalan, 36. They went missing on 12 March near Moyo River near the town of Tshimbulu. The body of their Congolese interpreter, Betu Tshintela, was discovered in the same location.

"I have no words except to thank you all for your support and prayers. Maybe words will come in time," John Sharp, who resides in Texas, wrote on his personal Facebook page.

Sharp, who was a coordinator with the United Nations Group of Experts for Congo, and his colleague had travelled to the Kasai-Central region to investigate summary executions of civilians and recently discovered mass graves when they were reported to have been kidnapped – alongside their Congolese translator and three local drivers.

Sweden's Prime Minister Stefan Löfven on 29 March issued a statement of sorrow and dismay at the murder of Catalan. "Zaida Catalan, employed by the UN as a humanitarian expert, was there to strengthen peace in the country. This task is crucial for creating greater security in the entire region and inspiring hope in a country long plagued by violence and destruction," Löfven, said.

"Zaida Catalan worked tirelessly for peace and justice, and risked her own life to save others. We mourn the fact that one more person has lost their life in the service of the UN and of peace; we share the sense of loss and despair with her family and friends, while also feeling a deep sense of gratitude for her work.

"We must now find out what happened to Zaida Catalan, her colleagues and the others affected. The UN and the Democratic Republic of the Congo must investigate this incident and hold those responsible to account. Sweden is naturally ready to assist in this work."

Zaida Catalan and Michael Sharp
United Nations investigators Zaida Catalan (L) and Michael Sharp (R) were kidnapped by unidentified men as they investigated reports of human rights violation in Kasai Central province, DRC Human Rights Watch

The French Mission to the UN on its Twitter account said: "France is very saddened by death of UN experts in DRC – we pay tribute to Michael Sharp and Zaida Catalan who were working for peace."

Matthew Rycroft, who is currently President of the UN Security Council and UK Ambassador and Permanent Representative to the UN, said on Twitter: "So sad that Michael Sharp and Zaida Catalan from UN Group of Experts died in DRC. They were committed to helping DRC people. RIP."

Posting a picture of Sharp smiling as he met with local villagers, Sasha Lezhnev associate director of policy at the watchdog non-governmental organisation Enough Project, also paid tribute to the American investigator. "Michael Sharp believed strongly in the mission of helping Congo achieve peace, a journey ended far too early. May his belief not die in vain," Lezhnev said in a tweet.

Pragmora, a conflict-resolution group that has focused on Congo issues, offered "its deepest condolences" to Sharp, Catalan and Tshintela's families. "We are so sorry for your loss," the organisation said on Twitter.

UN Secretary General, Antonio Guterres, on 29 March vowed to press for justice in the killing of the two experts and their interpreter. Guterres confirmed the UN would also conduct an inquiry into the murders.