A volunteer has been detained after the St Peter and St Paul cathedral in Nantes went up in flames on Saturday.
A Rwandan refugee who was in charge of locking up the cathedral the day before the fire has been taken for questioning to work out inconsistencies in his schedule.
But prosecutors said it would be "premature and hasty" to think that the man had any role in the fire, which destroyed the cathedral's stained glass windows and the grand organ before being put out.
"We must remain careful as to the interpretation of this police custody. It is a normal procedure," public prosecutor Pierres Sennès said.
The blaze is being treated as arson, after investigators worked out that three fires had been deliberately set in the cathedral. Police enquiries are ongoing.
The volunteer has not been named, but the cathedral's clerk said he was "a man of duty" with health problems and who "suffered a lot in Rwanda," which he left several years ago.
Jean-Charles Nowak told French newspaper Le Figaro: "I don't believe for a second that he could have set the cathedral on fire. It's a place he adores."
Around fire fighters rushed to stop the flames from destroying the main structure of the cathedral on Nantes. Prime Minister Jean Castex lauded them for their "professionalism, courage and self-control".
The fire comes over a year after a raging fire nearly destroyed Notre-Dame Cathedral in Paris.
French President Emmanuel Macron announced earlier in July that the spire of Notre-Dame would be rebuilt exactly as before - putting a stop to rumours that it would be changed to a more modern style.
