Smoke rises in front of the altar cross at Notre-Dame Cathedral in Paris on April 15, 2019, after a fire engulfed the building. - A huge fire swept through the roof of the famed Notre-Dame Cathedral in central Paris on April 15, 2019, sending flames and huge clouds of grey smoke billowing into the sky. The flames and smoke plumed from the spire and roof of the gothic cathedral, visited by millions of people a year. A spokesman for the cathedral told AFP that the wooden structure supporting the roof was being gutted by the blaze. (Photo by PHILIPPE WOJAZER / POOL / AFP)        (Photo credit should read PHILIPPE WOJAZER/AFP/Getty Images)
PHOTO: PHILIPPE WOJAZER/AFP/AFP/Getty Images
Smoke rises in front of the altar cross at Notre-Dame Cathedral in Paris on April 15, 2019, after a fire engulfed the building. - A huge fire swept through the roof of the famed Notre-Dame Cathedral in central Paris on April 15, 2019, sending flames and huge clouds of grey smoke billowing into the sky. The flames and smoke plumed from the spire and roof of the gothic cathedral, visited by millions of people a year. A spokesman for the cathedral told AFP that the wooden structure supporting the roof was being gutted by the blaze. (Photo by PHILIPPE WOJAZER / POOL / AFP) (Photo credit should read PHILIPPE WOJAZER/AFP/Getty Images)
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The steeple of the landmark Notre-Dame Cathedral collapses as the cathedral is engulfed in flames in central Paris on April 15, 2019. - A huge fire swept through the roof of the famed Notre-Dame Cathedral in central Paris on April 15, 2019, sending flames and huge clouds of grey smoke billowing into the sky. The flames and smoke plumed from the spire and roof of the gothic cathedral, visited by millions of people a year. A spokesman for the cathedral told AFP that the wooden structure supporting the roof was being gutted by the blaze. (Photo by Geoffroy VAN DER HASSELT / AFP)        (Photo credit should read GEOFFROY VAN DER HASSELT/AFP/Getty Images)
PHOTO: GEOFFROY VAN DER HASSELT/AFP/AFP/Getty Images
The steeple of the landmark Notre-Dame Cathedral collapses as the cathedral is engulfed in flames in central Paris on April 15, 2019. - A huge fire swept through the roof of the famed Notre-Dame Cathedral in central Paris on April 15, 2019, sending flames and huge clouds of grey smoke billowing into the sky. The flames and smoke plumed from the spire and roof of the gothic cathedral, visited by millions of people a year. A spokesman for the cathedral told AFP that the wooden structure supporting the roof was being gutted by the blaze. (Photo by Geoffroy VAN DER HASSELT / AFP) (Photo credit should read GEOFFROY VAN DER HASSELT/AFP/Getty Images)
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Smoke rises in front of the altar cross at Notre-Dame Cathedral in Paris on April 15, 2019, after a fire engulfed the building. - A huge fire swept through the roof of the famed Notre-Dame Cathedral in central Paris on April 15, 2019, sending flames and huge clouds of grey smoke billowing into the sky. The flames and smoke plumed from the spire and roof of the gothic cathedral, visited by millions of people a year. A spokesman for the cathedral told AFP that the wooden structure supporting the roof was being gutted by the blaze. (Photo by PHILIPPE WOJAZER / POOL / AFP)        (Photo credit should read PHILIPPE WOJAZER/AFP/Getty Images)
PHOTO: Philippe Wojazer/AFP/Getty Images
Smoke rises in front of the altar cross at Notre-Dame Cathedral in Paris on April 15, 2019, after a fire engulfed the building. - A huge fire swept through the roof of the famed Notre-Dame Cathedral in central Paris on April 15, 2019, sending flames and huge clouds of grey smoke billowing into the sky. The flames and smoke plumed from the spire and roof of the gothic cathedral, visited by millions of people a year. A spokesman for the cathedral told AFP that the wooden structure supporting the roof was being gutted by the blaze. (Photo by PHILIPPE WOJAZER / POOL / AFP) (Photo credit should read PHILIPPE WOJAZER/AFP/Getty Images)
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Smoke billows as fire engulfs the spire of Notre Dame Cathedral in Paris, France April 15, 2019. REUTERS/Benoit Tessier
PHOTO: Benoit Tessier/Reuters
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A woman reacts as she watches the flames engulf the roof of the  Notre-Dame Cathedral in Paris on April 15, 2019. - A huge fire swept through the roof of the famed Notre-Dame Cathedral in central Paris on April 15, 2019, sending flames and huge clouds of grey smoke billowing into the sky. The flames and smoke plumed from the spire and roof of the gothic cathedral, visited by millions of people a year. A spokesman for the cathedral told AFP that the wooden structure supporting the roof was being gutted by the blaze. (Photo by Geoffroy VAN DER HASSELT / AFP)        (Photo credit should read GEOFFROY VAN DER HASSELT/AFP/Getty Images)
PHOTO: Geoffroy Van Der Hasselt/AFP/Getty Images
A woman reacts as she watches the flames engulf the roof of the Notre-Dame Cathedral in Paris on April 15, 2019. - A huge fire swept through the roof of the famed Notre-Dame Cathedral in central Paris on April 15, 2019, sending flames and huge clouds of grey smoke billowing into the sky. The flames and smoke plumed from the spire and roof of the gothic cathedral, visited by millions of people a year. A spokesman for the cathedral told AFP that the wooden structure supporting the roof was being gutted by the blaze. (Photo by Geoffroy VAN DER HASSELT / AFP) (Photo credit should read GEOFFROY VAN DER HASSELT/AFP/Getty Images)
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French Prime Minister Edouard Philippe (L), and French President Emmanuel Macron (3rd L) gather in near the entrance of the Notre-Dame de Paris Cathedral in Paris, as flames engulf its roof on April 15, 2019. - A huge fire swept through the roof of the famed Notre-Dame Cathedral in central Paris on April 15, 2019, sending flames and huge clouds of grey smoke billowing into the sky. The flames and smoke plumed from the spire and roof of the gothic cathedral, visited by millions of people a year. A spokesman for the cathedral told AFP that the wooden structure supporting the roof was being gutted by the blaze. (Photo by PHILIPPE WOJAZER / POOL / AFP)        (Photo credit should read PHILIPPE WOJAZER/AFP/Getty Images)
PHOTO: Philippe Wojazer/AFP/Getty Images
French Prime Minister Edouard Philippe (L), and French President Emmanuel Macron (3rd L) gather in near the entrance of the Notre-Dame de Paris Cathedral in Paris, as flames engulf its roof on April 15, 2019. - A huge fire swept through the roof of the famed Notre-Dame Cathedral in central Paris on April 15, 2019, sending flames and huge clouds of grey smoke billowing into the sky. The flames and smoke plumed from the spire and roof of the gothic cathedral, visited by millions of people a year. A spokesman for the cathedral told AFP that the wooden structure supporting the roof was being gutted by the blaze. (Photo by PHILIPPE WOJAZER / POOL / AFP) (Photo credit should read PHILIPPE WOJAZER/AFP/Getty Images)
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TOPSHOT - Flames and smoke are seen billowing from the roof at Notre-Dame Cathedral in Paris on April 15, 2019. - A fire broke out at the landmark Notre-Dame Cathedral in central Paris, potentially involving renovation works being carried out at the site, the fire service said.Images posted on social media showed flames and huge clouds of smoke billowing above the roof of the gothic cathedral, the most visited historic monument in Europe. (Photo by Patrick ANIDJAR / AFP)        (Photo credit should read PATRICK ANIDJAR/AFP/Getty Images)
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PHOTO: ERIC FEFERBERG/AFP
Crowds look on as flames and smoke billow from the roof at Notre-Dame Cathedral in Paris on April 15, 2019. - A fire broke out at the landmark Notre-Dame Cathedral in central Paris, potentially involving renovation works being carried out at the site, the fire service said.Images posted on social media showed flames and huge clouds of smoke billowing above the roof of the gothic cathedral, the most visited historic monument in Europe. (Photo by ERIC FEFERBERG / AFP)
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(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.

The first alarm rang ‪at 6:15 p.m. local time. An alert flashed on the Notre Dame security control screens showing which sensor was sending the alert.

According to the police source, the two security agents went to check the location but didn’t see anything at that time. When the second alarm rang ‪at 6:43 p.m. local time, the two security agents followed the alarm system instructions and went to another location at the top of the cathedral, where they then saw the flames.

Prosecutors are investigating if a short-circuit caused the fire

Paris prosecutors are investigating if an electrical short-circuit caused the fire that ripped through the cathedral.

A judicial source told CNN on Thursday that “we are not excluding any hypothesis at this stage,” saying that this is not their only line of inquiry.

The cathedral was undergoing renovations at the time of the fire. Some scrutiny has fallen on the firms undertaking work on the 150-year-old spire, which collapsed Monday as the flames raged around it.

Of the four companies contracted to carry out renovations at Notre Dame, two companies – scaffolding firm Europe Echafaudage and art conservationists Socra – had work in progress there at the time of the fire. Neither company had workers on site when the fire broke out.

The fire devastated large parts of the 850-year-old building before it was finally extinguished after a nine-hour battle.

Scores of priceless artifacts were rescued from the flames, and were taken to the Louvre museum for safekeeping. The bees that live on the roof of Notre Dame also survived the devastating fire, beekeeper Nicolas Geant told CNN.

The Paris fire service said the operation was one of the most complex it had ever undertaken, and at one point, it was feared that the entire structure might be lost.

CNN’s Saskya Vandoorne, Kara Fox and Gianluca Mezzofiore contributed to this report.