Police have released a volunteer who worked at the fire-stricken cathedral after being brought into custody for questioning.
A volunteer operating at Nantes' fire-stricken St Peter and St Paul Cathedral who was taken into custody by police for questioning has been released.
A blaze ripped through the gothic cathedral early on Saturday, severely damaging its 17th century organ and stained glass windows.
The man, 39, who "was responsible for closing the cathedral on Friday evening," according to the city's public prosecutor Pierre Sennès, was cleared of all involvement and released on Sunday.
Speaking on behalf of his client, the man's lawyer Quentin Chabert said: "There is nothing that directly links my client to the fire at the cathedral."
Police called his detainment "a normal procedure."
The volunteer has not been named.
Sennès also revealed that the cause for the blaze was arson with "three distinct fire points" found.
The venue remains cordoned off as prosecutors have launched an investigation into the incident.
More than 100 firefighters were deployed in the western French city on Saturday morning to tackle a "violent blaze" at the cathedral.
Around 10.00 local time, General Laurent Ferlay told reporters the fire was under control.
Footage released on social media earlier by the French Federation of Firefighters showed flames leaping inside the gothic monument as a large cloud of black smoke escaped to rise towards the sky.
Emergency services were alerted shortly before 07.45 CEST and some 104 firefighters were deployed to tackle the fire, Ferlay said.
The "violent blaze" erupted "by the organ located behind the rose window", Ferlay explained.
"The roof has not been affected", he went on, adding: "the scenario is not comparable to Notre Dame".
This will be a relief for most French people still reeling from the devastating blaze 15 months earlier at Notre Dame cathedral in central Paris.
Contrary to the 850-year-old monument in the capital, which still had timber roof beams, the Cathedral of St. Peter and St Paul of Nantes had been renovated with concrete roof beams following a large fire in 1972 which had reduced the roof to ashes.
The renovation was only completed in 2013.
Per Ferlay, "damage is concentrated on the great organ which seems to be completely destroyed". The platform on which it stood is also "unstable", he added.
The great organ has been in the cathedral since 1621. It originally had only 27 keys but subsequent restoration works increased the number to 74.
It had survived the French Revolution, WWII bombings and the devastating 1972 fire.
A fire expert from the technical and scientific police laboratory in Paris is to be dispatched to Nantes to help with the investigations.
Nantes Mayor Johanna Rolland praised the fire service for their reactivity and professionalism.
President Emmanuel Macron expressed his support to firefighters "who take great risk to save this gothic gem in the city of the Dukes".
Prime Minister Jean Castex said on Twitter that his thoughts are "with our firefighters courageously mobilised to contain the fire".
"To the people of Nantes, whose emotion I share, I want to express my solidarity", he also wrote.
Construction of the cathedral, which began in 1434, took more than 450 years. The gothic monument was listed in 1862 before it was even finished.