Paris (CNN) Investigators have pinpointed the start of the fire that ripped through Notre Dame Cathedral on Monday, a police source told CNN.

After police reviewed a cache of amateur video and photos from local residences, investigators believe the fire started at the center of the cathedral's roof towards the base of the iconic spiral, the police source said.

The police source also told CNN that investigators are focusing on two potential problems relating to the outbreak of the fire. One issue under investigation is a possible problem with the fire alarm system sensors.

Investigators' second line of inquiry, the source said, relates to the elevators that were built for the construction workers. The source provided no additional details on that.

Two security guards at the cathedral told police they noticed the flames for the first time only when the flames were already almost 3 meters high ‪at 6:43 p.m. local time (12:43 p.m. ET), indicating the fire alarm sensors might not have been working properly, the police source told CNN.

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