How was it that fire ravaged Notre-Dame? And how was it saved?

A New York Times investigation found that a miscommunication among security employees and flawed planning let the fire spin out of control in one of France’s most revered monuments on April 15. When the firefighters were called, it was already too late to stop it from spreading.

Some 500 firefighters battled the blaze into the night. Some, at the peril of their own lives, went inside the northern tower to protect it from flames at a moment when it could have collapsed at any time. This decisive moment saved the structure.

Here is what the investigation revealed about a foretold tragedy, and how the worst was averted.

Security employees couldn’t locate the fire and waited 30 minutes before calling the firefighters.

After the first warning light flashed with the word “Feu,” or fire, at 6:18 p.m. on April 15, a fire security employee monitoring a control panel at the cathedral radioed a church guard by walkie-talkie, so he could go to check if there was actually a fire. This is a general French practice.

But the church guard went to the wrong place: Instead of walking up to the cathedral’s attic, he went to check the attic of an adjacent building, the sacristy. It is unclear whether the security employee misread the message, or if the church guard misunderstood it, but neither could locate the fire.