Asked whether the United Nations had done enough to prepare and protect the experts, José Luis Díaz, a spokesman, said the organization was asking itself the same question.

“This is why the U.N. is looking at the whole sequence of events in the disappearance and murder of our colleagues,” he said in an email. “We want to assess whether things worked as they should, and if what we have in place is adequate or robust enough.”

Congo has a long history of conflict and suffering. Foreign powers, successive leaders and an alphabet soup of rebel groups have all pillaged its rich natural resources. The country is the focus of the most expensive peacekeeping operation in the world. But the mission, called Monusco, has often been criticized for turning a blind eye to human rights abuses committed by government forces and rebels alike.

Ms. Catalán, a former Green Party activist in Sweden who had been working as a United Nations expert for less than a year, quickly got sucked into an extraordinarily dangerous world that she was woefully unprepared for, where the line between murderous rebels and corrupt politicians often blurs.

She worked assiduously to untangle a murky web of local politicians, rebel leaders and government ministers, trying to pin down perpetrators so that the Security Council could impose sanctions on them. Her tools often boiled down to a pen that doubled as a recorder, and a determined line of inquiry that ended up putting her life in danger.

It is still unclear who ordered the murders of the experts. The Congolese government said it had released the cellphone video to show that militia fighters, not its soldiers, were responsible. In April, the government announced the arrests of two men. One escaped. Then, on Saturday, the Congolese said they knew who had ordered the killings and where Ms. Catalán’s head was, but gave no further details.

But according to documents kept on Ms. Catalán’s computer and to others familiar with the case, she had been scrutinizing a government minister, Clément Kanku, for his possible role in inciting violence in the Congolese region of Kasai last year.