The suspect told the examining magistrate he lit three fires in the cathedral, which resulted in the severe damage of a 17th-century organ and stained glass windows.
He could now be handed “a 10-year prison sentence and a fine of €150,000 euros,” Nantes prosecutor Pierre Sennès explained.
The rector of Nantes cathedral, Father Hubert Champenois, said last week that the volunteer was a Rwandan citizen who came to France as a refugee “a few years ago”.
He said he had been serving “as an altar” and had known him for “four or five years”.
“Most of the works” have been saved from the fire and are now being stored in the castle of Nantes, said Philippe Charron, head of the heritage department at the Regional Directorate of Cultural Affairs (DRAC). But “very few” elements of the organ, “if any at all, will be saved”, he added.
Authorities are now assessing the reconstruction process “stone by stone”.
Prime Minister Jean Castex, who had come to Nantes to praise firefighters work on the day of the fire, said the state will fully take part to the works.
President Emmanuel Macron too had praised the effort of the emergency services to save “the jewel of the city of the Dukes”.
The Gothic building’s construction dates back to 1434 but works were completed only in 1891.
After a huge fire in 1972 that destroyed the roof, the cathedral had been renovated with concrete roof beams.
The renovation was only completed in 2013.