It was too early to estimate the cost of the damage, said the heritage charity Fondation du Patrimoine.

Paolo Violini, a restoration specialist for Vatican museums, said the pace at which the fire spread through the cathedral had been stunning.

"We are used to thinking about them as eternal simply because they have been there for centuries, or a thousand years, but the reality is they are very fragile," Violini said.

HUMAN CHAIN

The company carrying out the renovation works when the blaze broke out said it would cooperate fully with the investigation.

"All I can tell you is that at the moment the fire began none of my employees were on the site. We respected all procedures," Julien Le Bras, a representative of family firm Le Bras Freres.

Officials breathed a sigh of relief that many relics and artworks had been saved. At one point, firefighters, policemen and municipal workers formed a human chain to remove the treasures, including a centuries-old crown of thorns made from reeds and gold, and the tunic believed to have been worn by Saint Louis, a 13th century king of France.

"Notre-Dame was our sister, it is so sad, we are all mourning," said Parisian Olivier Lebib. "I have lived with her for 40 years. Thank God that the stone structure has withstood the fire."

Xinhua and agencies contributed to this story.