French investigators were on Sunday questioning a man who worked at the cathedral in the city of Nantes which was badly damaged by fire a day earlier, a prosecutor said.
The man "was responsible for locking up the cathedral on Friday evening and investigators wanted to clarify elements of the schedule of this person", prosecutor Pierre Sennes told AFP.
But he emphasised that "any interpretation that could implicate this person in what occurred is premature" adding the questioning was part of "normal procedure".
Prosecutors launched an arson investigation after the fire broke out on Saturday morning in three places at the gothic Cathedral of St Peter and St Paul.
The blaze destroyed stained glass windows and the 17th century grand organ — a star attraction of the cathedral.
About 100 firefighters saved the main structure, built between the 15th and 19th centuries.
Sennes confirmed that experts from a police unit specialised in fire investigations were at the scene.
They were awaiting authorisation from firefighters to examine the platform on which the grand organ had stood.
Cathedral rector Father Hubert Champenois said Saturday "everything was in order last night," and that "a very close inspection was made before it closed, like every other evening."
The building was last hit by fire in 1972 and its roof took more than 13 years to repair.
Regional fire chief Laurent Ferlay said Saturday the damage was not comparable to the 1972 blaze, or to last year's devastating blaze at Notre-Dame cathedral in Paris.
Much of Notre-Dame's roof and wooden structure was destroyed, its steeple collapsed and fumes containing toxic molten lead billowed into the air.
