Notre Dame havde brandvarsling, men ikke sprinkleranlæg og brandskillevægge i den mest udsatte del af loftet, kaldt “skoven” pga. tætheden af tagbjælker. Det var her den første alarm gik kl. 18.20.

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.

1843 gik den anden alarm og nu gik det hurtigt. Et loft kan virke som en vindtunnel. Det er derfor man bygger brandskillevægge. Men det blev ikke gjort i Notre Dame af hensyn til restaureringen og fordi man ikke ønskede at trække elkabler i det brandfarlige miljø.

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.

Hvad der forårsagede branden, ved man endnu ikke. Alle muligheder holdes åbne.

Men det synes klart at bevaringshensyn kan være sket på bekostning af brandsikkerheden.

Much remains to be learned. But already it is emerging that Notre-Dame, irreplaceable as it is to France’s heritage, lacked the fundamental fire-prevention safeguards that are required in more modern structures and have been grafted onto other ancient cathedrals elsewhere in Europe.

Some of those elements, like firewalls or a sprinkler system, were absent by choice — so as not to alter the landmark’s design or to introduce electrical wiring deemed a greater risk amid the timbers that supported Notre-Dame’s ornate lead roof.

“There had been a systematic refusal to install anything electrical” within “the forest” because of the risk, said Pierre Housieaux, president of the Paris Historical Association. “Everyone knew that the attic was the most fragile part.”

Det gjorde at branden fik lov til at udvikle sig. Flammerne slikkede op over spiret som blev de rene skorstenspiber.

“The fire-detection system existed, not the fire compartments,” said Jacques Chanut, president of the French Building Federation, referring to the structures commonly used elsewhere to contain blazes. “That’s the typical example of something we are going to have to think about tomorrow.”

Notre-Dame Attic Was Known as ‘the Forest.’ And It Burned Like One.