Pope Francis said on Wednesday that he was greatly pained by the killing of a French priest, allegedly by a Rwandan immigrant already under investigation for setting the Nantes cathedral on fire last year.

Father Olivier Maire, 60, head of the Montfortain Missionary Order at Saint-Laurent-sur-Sevre, was found dead in a room of the abbey.

"It was with great pain that I learned of the murder ...", Francis told French tourists and pilgrims at his weekly general audience.

He expressed his condolences to the members of the order, to the priest's family and to all French Catholics.

"I assure you of my participation (in the mourning) and my spiritual closeness," he said.

The 40-year-old Rwandan surrendered to police on Monday and admitted to killing the head of the religious congregation where he was staying as he awaited trial for the cathedral fire.

He was arrested on suspicion of murder, prosecutors said.

La Roche-sur-Yon prosecutor Yannick Le Goater told reporters police had found Maire's body in a room to which the suspect had given them the key.

Following the cathedral fire in July 2020, the suspect was held in detention until late May when he was then released under judicial supervision and placed in the abbey, the prosecutor said.

After talking about wanting to leave, he was transferred to a psychiatric hospital where he stayed until the end of July.