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.”

“The investigation is going to be long and complex,” he added. “We are in the process of interviewing witnesses.”