Notre Dame officials were alerted to a fire 23 minutes before but "found no sign of a fire"

PARIS—An alarm was raised at Notre Dame at 6:20 p.m. on Monday night—23 minutes before the structure was engulfed in flames—but officials found no sign of a fire.

Firefighters who responded to a second alert raced to the scene but were then unable to tame an inferno that ripped through the 12th century cathedral for the next 9 hours.

Paris public prosecutor Rémy Heitz announced on Tuesday that a full investigation would uncover how a massive fire was allowed to gut the cathedral.

“What we know at this stage is that there was an initial alarm at 6:20 p.m., followed by a procedure to verify this but no fire as found,” Heitz explained. “Then, there was a second alarm at 6:43 p.m. and at that point a fire was detected in the structure.”

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