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.
"Everyone was immobilized by shock for maybe a minute," Vexo said. Within minutes, parishioners and tourists emptied the cathedral, leaving just staff behind.
When he left the building about 20 minutes later, Vexo said there was still no evidence of a fire, adding that he thought it was a false alarm.
Monsignor Patrick Chauvet, the rector of Notre Dame, told French radio station France Inter that the cathedral has fire supervisors who go up to the attic to examine the frame three times a day.
"In terms of security, I doubt we could have done more," Chauvet said.
Heitz said there were no indications that the blaze was started deliberately. Investigators plan to interview representatives from the companies that were renovating the structure at the time of the fire.
Meanwhile, French President Emmanuel Macron has vowed to rebuild the cathedral as billionaires and corporations step forward to pledge financial donations to support that effort.