Notre Dame Cathedral could have burned down completely had Paris firefighters not acted as quickly as they did, a French government official said Wednesday. The 850-year-old landmark would have burned to the ground in a “chain-reaction collapse,” according to José Vaz de Matos, a fire expert with France’s Culture Ministry. He said the measures taken to protect the wooden supports in the twin medieval bell towers averted a bigger catastrophe.“If the fire reached this wooden structure, the bell tower would have been lost,” de Matos said at a news conference. “From the moment we lose the war of the bell towers, we lose the cathedral, because it’s a chain-reaction collapse.”

The fire destroyed most of the building’s roof and caused its spire to collapse. A fire alarm was first sounded 23 minutes before the blaze was discovered as Mass was underway in the cathedral. The blaze was finally discovered on the roof. Firefighters and church officials speedily evacuated those inside, and no one was killed.