Three Italian nuns were found killed, two of them raped and decapitated, over the weekend in the north of Burundi's capital, officials and a priest in the African state said on Monday.
Police said three suspects had been detained for questioning. A motive was not immediately clear.
Father Mario Pulicini, who is responsible for the parish in a northern suburb of Bujumbura, named two of the nuns as Lucia Pulici, who was 75 and due to celebrate her birthday on Monday, and Olga Raschietti, 82. He said they were found decapitated in their dormitory on Sunday.
The third nun, 79-year-old Bernadetta Boggian, was found dead early on Monday morning, he told Reuters. The killings appeared to have happened at two separate times, he said.
"It is very difficult to know the reason behind the killing, but nothing can justify it," Father Mario said.
Evidence showed that two of the nuns had been raped before they were killed, police spokesman Hermenegilde Harimenshi said. They had been "partially decapitated," Harimenshi said.
Italy's Foreign Ministry also reported the three murders. The Vatican said Pope Francis was "greatly saddened" by the killings.
"Pope Francis has learned with great sadness of the murder of three nuns," Vatican Secretary of State Cardinal Pietro Parolin said in a telegram sent on the pope's behalf.
The pope offered condolences in the "tragic death" of the nuns to their families, the local parishioners and the sisters' order, the Xavarian Missionary Sisters of Mary.
The Catholic diocese in Parma, Italy, said on its website that the death of Pulici and Raschietti appeared to have been "the tragic outcome of an armed robbery by a mentally unbalanced person".
The two nuns had served in Burundi for seven years, after working several years in the east of another central African state, the Democratic Republic of Congo.