The 33-year-old, who has not been named, on Tuesday confessed to local police that he had raped and murdered three Roman Catholic nuns at a convent in Bujumbura, the capital of Burundi.
“I raped them then I killed them,” he reportedly told police, according to Tgcom24, because “they are foreigners who are occupying my property.”
The man was found with a key to the convent and a mobile telephone belonging to one of the nuns, according to police sources.
Police said Lucia Pullici, 75, and Olga Raschietti, 83, were stabbed to death on Sunday afternoon, with the killer then battering one of the two with a rock.
Hours later, before dawn on Monday, another nun, Bernardetta Boggian, aged 79, was killed in the same convent, her body beaten and her head hacked off.
It is still not clear how the killer managed to enter the convent and kill the third nun without being stopped by the police at the scene.
Initial reports said the man who carried out the first attack, killing two nuns on Sunday afternoon, fled the scene.
According to Tgcom24, the alleged killer spent a large part of his life in Congo and returned to Burundi eight years ago.
Pope Francis on Monday issued his condolences."The Holy Father begs the Lord to welcome into his kingdom of peace and light these three faithful and devout nuns," he said in a statement.
Burundian Vice-President Prosper Bazombanza has said the government was "appalled by such barbarity".
In 2011, a Croatian nun and an Italian charity worker were killed in an apparent botched robbery in northern Burundi.
The small nation in Africa's Great Lakes region emerged in 2006 from 13 years of brutal civil war and its political climate remains fractious ahead of presidential polls due in June 2015.