A French judicial police official said investigators think an electrical short-circuit most likely caused Notre Dame Cathedral fire.
The official, who spoke anonymously about the ongoing investigation, said investigators still don't have the green light to work in the cathedral and search in the rubble for safety reasons.
He told The Associated Press the monument is still being consolidated with wooden planks to support some fragilize parts of the walls.
-By Nicolas Vaux-Montagny
An association of Polish rabbis says that they are "broken hearted" over the damage inflicted by fire on Notre Dame Cathedral.
In a letter sent on Thursday to Polish bishops, they said "We as Jews know what it means to see your house of worship burning. We still fast every year on the anniversary of the burning of the Temple in Jerusalem 2,000 years ago. We, here in Poland, also remember the burning and destructions of our synagogues during WWII."
The rabbis went on to say that "these experiences have made us very sensitive and aware of the pain and horror of the destruction of any house of worship."
Archbishop Stanislaw Gadecki, the head of the Polish Roman Catholic episcopate, thanked Michael Schudrich, Chief Rabbi of Poland, to thank him for the letter and the support.
The rector of Notre Dame Cathedral has proposed building a small, temporary church on the plaza outside the monument so that the faithful has a place of worship while the 12th-century structure ravaged by fire is closed for restoration.
Bishop Patrick Chauvet offered the idea of creating an "ephemeral" cathedral so people can pray and life can return to normal in the neighborhood.
The Paris City Hall gave its approval to the idea Thursday "subject to technical restraints."
A crypt containing vestiges dating from antiquity is located under the vast esplanade. There is also an underground parking lot.
President Emmanuel Macron has said he wants to see the cathedral fully restored in five years. Chauvet told neighborhood business owners on Wednesday he expected Notre Dame to remain closed "five to six years."
The proceeds from a new classical music album will be donated to the heritage foundation in France to help support the restoration of Notre Dame Cathedral.
The album, entitled simply "Notre-Dame," a collection of sacred music, is being released by Universal Music France on digital platforms Friday.
Many tracks were recorded at the cathedral and they include artists Cecilia Bartoli, Jessye Norman, Roberto Alagna, Christopher Hogwood and Herbert von Karajan.
Two of Notre Dame's most recent organists Pierre Cochereau and Olivier Latry also feature.
The cathedral's famous 8000-pipe organ survived the blaze, which destroyed most of the roof and caused the spire's collapse.
Universal Music France President Olivier Nusse said: "We had to help the cathedral once again stand as a symbol of French culture across the world."
French President Emmanuel Macron has thanked hundreds of firefighters who helped save Notre Dame Cathedral.
During a ceremony at the Elysee presidential palace in the presence of those who intervened on Monday's fire, Macron praised their courage and exemplarity ability to make the right decisions under time pressure.
"We've seen before our eyes (your) responsibility, courage, solidarity and a meticulous organization", Macron said. "The worst has been avoided."
Macron said firefighters will get an Honor Medal for their courage and devotion.
About 500 firefighters took part in the nine-hour battle to save the 12th-century Notre Dame on Monday evening. Its spire collapsed and roof was destroyed, but its iconic towers, rose windows, famed organ and precious artworks were saved.
For some Britons who watched Notre Dame in Paris burn so fiercely, horror was mixed with apprehension. What happened to the French landmark this week could befall the Houses of Parliament in the future.
The seat of Britain's government is a crumbling, leaky, rodent-infested building with a series of rooms that make it a known fire trap. A 2016 report warned that the building is at risk of a "catastrophic event" such as a flood or fire that would leave it uninhabitable.
British lawmakers have agreed to move out for several years so the building on the River Thames can be repaired, but not until the mid-2020s. For now, fire wardens patrol it 24 hours a day.
Parliament's Restoration and Renewal Program says authorities "stand ready to learn any lessons that emerge from the fire at Notre Dame" to protect the building and its occupants.
The chaplain of the Paris fire brigade credits saving the crown of thorns from fire that gutted Notre Dame to a team of rescuers who broke the relic's protective covering and an official who had the secret code to unlock the protection.
Fire chaplain Jean-Marc Fournier told France Info on Thursday that his own team arrived on the heels of the salvaging, and praised the action "to preserve this extraordinary relic, this patrimony of humanity."
Others had earlier credited Fournier with saving the crown of thorns.
Fournier told daily Le Parisian that he himself was able to save the most precious thing for Catholics from the fires, the concentrated hosts. The paper said he climbed on altars to remove large paintings, but that he felt especially proud of another personal salvaging operation: "to have removed Jesus" from the Cathedral.
He was referring to the chalice containing consecrated hosts that for Catholics are the body of Christ.
Workers are putting wooden planks above one of Notre Dame's famed rose windows to give it more support after this week's devastating fire.
A huge crane and renovation teams worked at the site Thursday, after authorities warned that some of the structure remains at risk. Firefighters walked on what the remains of the roof to inspect damage.
The island housing Notre Dame at the heart of the French capital remained largely empty Thursday and closed to everyone but residents. Businesses were shuttered and the usual tourist throngs were nowhere to be seen.
Passersby praised the French firefighters who helped save the overall structure of the cathedral, although its spire collapsed and its roof was destroyed in Monday's devastating fire.
Benedicte Contamin, who came to see the cathedral Thursday, said she's sad but grateful it's still there. She said this is "a chance for France to bounce back, a chance to realize what unites us, because we have been too much divided over the past years."
France is paying a daylong tribute Thursday to the Paris firefighters who saved the internationally revered Notre Dame Cathedral from collapse and rescued its treasures from encroaching flames.
French President Emmanuel Macron will host the firefighters for a special gathering to share "words of thanks." Top government ministers will also take part in the event at the presidential palace in Paris.
Later, Paris City Hall will hold a ceremony in the firefighters' honor, with a Bach violin concert, two giant banners strung from the monumental city headquarters and readings from Victor Hugo's "The Hunchback of Notre Dame."
More than 400 firefighters took part in the nine-hour battle to save the 12th-century Notre Dame on Monday evening. Its spire collapsed and roof was destroyed, but its iconic towers, rose windows, famed organ and precious artworks were saved.
Remarkably, no one was killed in the fire, which occurred during a Mass, after firefighters and church officials speedily evacuated those inside.
Read and watch all AP coverage of the Notre Dame fire at https://apnews.com/NotreDameCathedral
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