Pope Francis is 'greatly saddened' by attack and has sent his condolences

The nun who found them and raised the alarm was also found dead later

Two were found 'partially decapitated' and raped in their dorm on Sunday

They had been at Roman Catholic convent in Kamenge, Burundi, for 7 years

Three elderly Italian nuns were raped and beaten before two were decapitated and another murdered in a convent in Burundi.

The bodies of Bernardetta Boggia, 79, Lucia Pulici, 75, and Olga Raschietti, 82, were found in their dormitory in Kamenge, north of the capital of Bujumbura.

Police said three suspects had been detained for questioning as they probe claims it was a botched robbery at the hands of a mentally unbalanced attacker.

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Horrific: Lucia Pulici (left) and Olga Rachietti (right) were raped and decapitated in their convent in Burundi

Murdered: Bernadette Boggia, 79, (left) was also killed after finding her fellow missionaries slain in their room

Father Mario Pulicini, who is responsible for the parish in a northern suburb of Bujumbura, said Ms Pulici and Ms Raschietti were found 'partially decapitated' in their dormitory on Sunday. It appeared all three Italian nuns had been raped.

Ms Pulici had been celebrate her 76th birthday on Monday.

The third nun, Ms Boggia, found the pair and raised the alarm.

But early on Monday morning other missionaries heard suspicious noises and rushed to find Ms Boggia also dead in her room.

Video courtesy of Rome Reports

Devastated: Colleagues and the Vatican are mourning the loss as police probe claims it was a botched robbery

Attack: The attacker appears to have broken into the Roman Catholic convent during the night on Sunday

Grief: The missionaries at the convent (pictured) had been due to celebrate Ms Pulici's 76th birthday on Monday

'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 Roman Catholic nuns had been raped before they were killed, police spokesman Hermenegilde Harimenshi said.

The Vatican said Pope Francis was 'greatly saddened' by the killings of the women, who had worked at the African convent for years. Ms Boggia had been there since the 1970s.

Dedicated: Ms Boggia pictured recently teaching children at the convent's school how to write

Investigation: Father Mario Pulcini, abbot of the parish, is in talks with police to help catch the attacker

'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'.