Julien le Bras' company has 12 workers involved in the refurbishment, though none were on site at the time of the fire. Le Bras insisted that "all the security measures were respected," and "workers are participating in the investigation with no hesitation."
Various officials have suggested the fire could have been linked to the renovation work. Paris prosecutor Remy Heitz said the investigation is in its early stages and is focusing on hearings while the site is being secured.
Queen Elizabeth II has sent a message of sympathy to French President Emmanuel Macron after a fire ravaged Notre Dame cathedral in Paris.
The British monarch says she was "deeply saddened" to see the cathedral ablaze, and expressed "sincere admiration to the emergency services who have risked their lives to try to save this important national monument."
British politicians and religious leaders have also sent messages of goodwill and offers of help in rebuilding the medieval building.
Archbishop of Canterbury Justin Welby, head of the Church of England, tweeted an image of the fire-damaged cathedral with a passage from the Bible: "The light shines in the darkness, and the darkness has not overcome it.'"
The Vatican's culture minister has offered words of hope to France following the devastating fire at Notre Dame, saying the cathedral is a "living creature" that has been reborn before and will continue to be the "beating heart" of France.
Cardinal Gianfranco Ravasi opened a Vatican press conference with a personal reflection on the cathedral. He noted it was a place of encounter for both believers and nonbelievers drawn to its beauty and in some cases, such as the 19th century French poet Paul Claudel, were converted to the Catholic faith as a result.
Ravasi, whose office oversees the patrimony of the Catholic Church worldwide, said he was moved by the scenes of faithful and tourists alike weeping as Notre Dame went up in flames.
He suggested that the Vatican, particularly its art experts at the Vatican Museums, could play a possible role in the rebuilding given their expertise.
The Paris prosecutor says there's no evidence of arson in the Notre Dame fire and that they're working on the assumption that the blaze was an accident.
Remy Heitz says the investigation will be "long and complex."
Speaking Tuesday, after the blaze was put out, he said 50 investigators are working on the probe. He says they will be interviewing workers from five companies that had been hired to work on renovations to the cathedral's roof, which was being repaired before the fire and which is where the flames first broke out.
This version corrects that 50 investigators are working on the probe, not five.
An aide says that Poland's president, Andrzej Duda, has offered assistance and Polish specialists for the task of rebuilding Paris' Notre Dame cathedral that was damaged by fire.
Krzysztof Szczerski said Tuesday that Duda has written to French President Emmanuel Macron to express Poland's grief and solidarity at thet loss of heritage and cultural identity.
He said that in a gesture of "European solidarity" Duda offered Poland's experience and world-class experts in the reconstruction of historic buildings. Warsaw and many other places were rebuilt from World War II rubble.
He said that a Polish chapel at the cathedral was affected by the fire but was not damaged.
A precious copy of Poland's most revered icon as well as relicts of Polish-born pope St. John Paul II have been rescued.
Germany's foreign minister says his country is prepared to help with the rebuilding of Notre Dame cathedral after a devastating fire.
Heiko Maas wrote on Twitter that French President Emmanuel Macron has called for help from outside France and "Germany stands ready to do that in close friendship."
Maas added that "we are united in sorrow. Notre Dame is part of the cultural heritage of mankind and a symbol for Europe."
Egypt's top Muslim cleric has expressed sadness over the fire that destroyed part of the famous Notre Dame cathedral in Paris, describing it as a "historic architectural masterpiece."
Sheikh Ahmed al-Tayeb, the grand imam of Al-Azhar, the Sunni Muslim world's seat of learning, wrote on Facebook: "Our hearts are with our brothers in France."
Paris' deputy mayor says Notre Dame's organ, among the world's most famous and biggest, remains intact after a devastating fire at Paris' main cathedral.
Emmanuel Gregoire told BFMTV Tuesday that a plan to protect Notre Dame's treasures was rapidly and successfully activated.
The impressive organ dates to the 1730s and was constructed by Francois Thierry. It boasts an estimated 8,000 pipes.
Gregoire also described "enormous relief" at the salvaging of pieces such as the purported Crown of Christ.
Egypt's Coptic Church has expressed "profound sadness" over the massive blaze that burned parts of the Notre Dame cathedral in Paris.
The head of Egypt's Copts, Pope Tawadroz II, said in a statement that the fire was a "huge loss for entire humanity" and affected "one of the most important monuments in the world."
The Foreign Ministry in Cairo also expressed "great regret and pain" over the fire, citing Notre Dame's "historical and culture value" for France and world heritage.
Pope Francis is praying for French Catholics and the Parisian population "under the shock of the terrible fire" that ravaged the Notre Dame cathedral.
Vatican spokesman Alessandro Gisotti said on Twitter on Tuesday that the pope "is close to France" and that he is offering prayers "for all those who are trying to cope with this dramatic situation."
The Vatican on Monday expressed "shock and sadness" at the fire that caused extensive damage to a cathedral that is "a symbol of Christianity in France and in the world."
Funding for the reconstruction of Notre Dame is piling up at a spectacular rate, with two of France's richest families together quickly pledging 300 million euros.
Businessman Francois-Henri Pinault and his billionaire father Francois Pinault said they were immediately giving 100 million euros from their company, Artemis, to help finance repairs to the cathedral devastated by fire Monday night.
A statement from Francois-Henri Pinault said: "This tragedy impacts all French people" and "everyone wants to restore life as quickly as possible to this jewel of our heritage."
That donation was then trumped by French tycoon Bernard Arnault and his luxury goods group LVMH, which pledged 200 million euros.
LVMH called the cathedral a "symbol of France, its heritage and its unity."
European Union chief Donald Tusk is calling on the bloc's member countries to help France rebuild the fire-ravaged Notre Dame cathedral saying the site in Paris is a symbol of what binds Europe together.
Tusk, who chairs summits of EU national leaders, told lawmakers Tuesday that the blaze reminds Europeans of "how much we can lose."
Tusk said: "At stake here is something more than just material help. The burning of the Notre Dame cathedral has again made us aware that we are bound by something more important and more profound than treaties."
Parliament President Antonio Tajani invited EU lawmakers, meeting in Strasbourg, France, to contribute their day's salary to help finance reconstruction.
A spokesman for Paris firefighters says that "the entire fire is out" at Notre Dame cathedral.
Gabriel Plus said Tuesday morning that emergency services are currently "surveying the movement of the structures and extinguishing smoldering residues."
Plus said that now the fire is out "this phase is for the experts" to plan how to consolidate the edifice.
French tycoon Bernard Arnault and his luxury goods group LVMH have pledged 200 million euros ($226 million) for the reconstruction of Notre Dame, following a reported 100 million-euro donation from another French billionaire, Francois Pinault.
A statement Tuesday from LVMH said the luxury goods group and the Arnault family would make the donation to a rebuilding fund for the cathedral, which was consumed by flames Monday evening.
LVMH called the cathedral a "symbol of France, its heritage and its unity."
The Pinault family's earlier 100 million-euro donation was widely reported by French media.
A French cultural heritage expert says France no longer has trees big enough to replace ancient wooden beams that burned in the Notre Dame fire.
Bertrand de Feydeau, vice president of preservation group Fondation du Patrimoine, told France Info radio that the wooden roof that went up in flames was built with beams more than 800 years ago from primal forests.
Speaking Tuesday, he said the cathedral's roof cannot be rebuilt exactly as it was before the fire because "we don't, at the moment, have trees on our territory of the size that were cut in the 13th century."
He said the restoration work will have to use new technologies to rebuild the roof.
Experts are assessing the blackened shell of Paris' iconic Notre Dame cathedral to establish next steps to save what remains after a devastating fire destroyed much of the almost 900-year-old building.
With the fire that broke out Monday evening and quickly consumed the cathedral now under control, attention is turning to ensuring the structural integrity of the remaining building.
Junior Interior Minister Laurent Nunez announced that architects and other experts would meet at the cathedral early Tuesday "to determine if the structure is stable and if the firefighters can go inside to continue their work."
Officials consider the fire an accident, possibly as a result of restoration work taking place at the global architectural treasure, but that news has done nothing to ease the national mourning.