PARIS — Inside the cavernous cathedral of Notre-Dame in Paris, the last Mass of the day was underway on Monday of Holy Week when the first fire alarm went off. It was 6:20 p.m., 25 minutes before the heavy wooden doors were scheduled to close to visitors for the day.

Worshipers, sightseers and staff were ushered out, and someone went up to check the most vulnerable part of the medieval structure — the attic, a lattice of ancient wooden beams known as “the forest” — but no fire was found, Rémy Heitz, the Paris prosecutor, said on Tuesday.

At 6:43 p.m., another alarm rang. It was just 23 minutes later, but when they returned to the attic, it was clear they had a major problem: It was on fire. Soon much of the roof and the delicate spire rising high above it were also engulfed in flames, fanned by a strong breeze.