A fire alarm first wailed inside the Notre Dame Cathedral at 6:20 p.m. Monday, but for 23 critical minutes cathedral staff searched for a blaze, unable to find the cause.

It wasn't until a second alarm went off at 6:43 p.m. that a fire was detected in the attic of the centuries-old religious landmark, French officials said Tuesday.

Paris prosecutor Rémy Heitz laid out a timeline of the night's events as his office opened an investigating into the cause of the catastrophic fire that tore through the historic structure, toppling its iconic spire and crumbling much of the monument's roof.



"We are favoring the theory of an accident," Heitz told reporters Tuesday, adding that the investigation into the blaze would be "long" and "complex."



Heitz said the initial alarm, which was looked into by cathedral staff and not firefighters, caused the church to be evacuated but did not lead to the discovery of the fire, according to the Washington Post.

"In the meantime, the church was evacuated because a Mass just started a bit earlier," Heitz said.

Johann Vexo, an organist at the Notre Dame who was inside the cathedral, told French newspaper Ouest-France that he didn't smell or see any smoke or flames when the first alarm went off.