Paris (CNN) Prosecutors investigating the causes of the inferno that tore through Notre Dame Cathedral in Paris have interviewed construction workers and security staff, as more details emerged about the effort to contain the blaze.

Ten people were interviewed by criminal investigators on Wednesday, the Paris Prosecutor's Office said, adding that interviews would continue on Thursday. On Tuesday, the prosecutor's office had said 30 employees at the Paris landmark had been questioned.

"Tomorrow new witnesses will be heard as well as people already interviewed for further information," the prosecutor's office said.

The prosecutor's office said that in addition to interviews, forensics teams and the central laboratory for the police department had been able to access some areas of the site and begin inspections. Officials are continuing to pursue the theory that the cause of the fire was accidental but have not ruled out other scenarios at this stage, the prosecutor's office added.

"While the prosecutor's office does not rule out any hypothesis, we remind that at this stage, nothing in the investigations highlights a criminal origin. Accidental causes remain our privileged lead," the prosecutor's office said.

The Paris fire service, meanwhile, said the nine-hour battle to save the building was one of the most complex it had ever undertaken. At one point, it was feared that the entire structure might be lost.

"If the flames had actually got to the timber frames of the belfries then we would have lost the cathedral completely because it would have led to a chain reaction of collapse," said Gabriel Plus, a spokesman for the fire service.

Philippe Demay, deputy chief of the Paris Fire Brigade, told reporters on Wednesday that the operation to put out the flames was "complex," and the "most complicated" he had ever encountered.

On Wednesday evening, cathedrals across France rang their bells in honor of Notre Dame, marking two days since the fire. Bells tolled at 6:50 p.m. across the country acknowledging what the French Bishops Conference described as "a shock that affects far beyond just the Catholics of our country."

The French government said it was committed to rebuilding the cathedral. Prime Minister Edouard Philippe announced an international architects' competition to replace the building's fallen spire.

As the scale of the damage became clear, investigators said they were determined to get to the bottom of how the blaze started. "Investigations continue to search for the truth and identify the origins and causes of the fire," the Paris Prosecutor's Office said.

Scrutiny has fallen on the firms undertaking the renovation of the 150-year-old spire, which collapsed Monday as the flames raged around it.

JUST WATCHED Notre Dame: Aerial animation shows the damage caused by fire Replay More Videos ... MUST WATCH Notre Dame: Aerial animation shows the damage caused by fire 00:37

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, CNN has been told.

According to centraledesmarches.com , a website where private companies can bid for public projects, Europe Echafaudage and its parent company, Le Bras Freres, were awarded contracts worth €3,493,766 ($3,951,396) for scaffolding and other services.

Julien Le Bras, CEO of Europe Echafaudage, which received a contract to renovate the spire in 2017, told reporters Tuesday that 12 of his employees were working on the project, but that "there were absolutely no workers on site when the fire broke out, and there hadn't been for quite a while before the fire."

Mark Eskenazi, a PR representative for Le Bras Freres, said the company is speaking to experts about how to take down the scaffolding and "absolutely" denies responsibility for what happened, saying that its workers had left the cathedral one hour before the fire began. He called the fire a "crisis" for the small business of 20 workers. "They are artisans, it is a very small enterprise."

Photos: In pictures: The fire that devastated Notre Dame Cathedral Notre Dame cathedral is seen on April 14, on the eve of the one year anniversary of the disastrous fire that ravaged the famous church. Hide Caption 1 of 47 Photos: In pictures: The fire that devastated Notre Dame Cathedral French Army General Jean-Louis Georgelin, head of Notre Dame Cathedral's reconstruction, speaks to journalists in Paris on April 14. Work at the Paris site has been suspended since March 16, when France introduced widespread measures to help control the spread of Covid-19. Hide Caption 2 of 47 Photos: In pictures: The fire that devastated Notre Dame Cathedral Notre Dame and the Pont de la Tournelle bridge are reflected in the Seine river on Easter Sunday, April 12. Hide Caption 3 of 47 Photos: In pictures: The fire that devastated Notre Dame Cathedral Notre Dame cathedral rector Patrick Chauvet repositions the Crown of Thorns, a relic of the passion of Christ, after a meditation ceremony to celebrate Good Friday in a secured part of Notre Dame on April 10. Hide Caption 4 of 47 Photos: In pictures: The fire that devastated Notre Dame Cathedral Professional mountain climbers work on a part of the Notre Dame Cathedral restoration on January 29. Hide Caption 5 of 47 Photos: In pictures: The fire that devastated Notre Dame Cathedral Notre Dame Cathedral is seen undergoing renovations on January 6. Hide Caption 6 of 47 Photos: In pictures: The fire that devastated Notre Dame Cathedral A carpet from Notre Dame cathedral undergoes restoration at 'the Mobilier National' headquarters in Paris on September 12, 2019. Hide Caption 7 of 47 Photos: In pictures: The fire that devastated Notre Dame Cathedral A protective net is seen installed at Notre Dame de Paris Cathedral during preliminary work on May 15, 2019, one month after the fire. Hide Caption 8 of 47 Photos: In pictures: The fire that devastated Notre Dame Cathedral Flowers are laid on a bridge in front of Notre Dame on April 17, 2019. Hide Caption 9 of 47 Photos: In pictures: The fire that devastated Notre Dame Cathedral Debris from the roof lies near the altar inside Notre Dame Cathedral in Paris on April 16, 2019, the day after the devastating fire. Hide Caption 10 of 47 Photos: In pictures: The fire that devastated Notre Dame Cathedral Daylight shines through the fallen roof of Notre Dame on April 16, 2019. Hide Caption 11 of 47 Photos: In pictures: The fire that devastated Notre Dame Cathedral The interior of Notre Dame is seen in this photo taken April 16, 2019. Hide Caption 12 of 47 Photos: In pictures: The fire that devastated Notre Dame Cathedral A man inspects damage near charred windows. Hide Caption 13 of 47 Photos: In pictures: The fire that devastated Notre Dame Cathedral Firefighters exit Notre Dame Cathedral on April 16, 2019. Hide Caption 14 of 47 Photos: In pictures: The fire that devastated Notre Dame Cathedral The burnt out cathedral is seen from the top of the Montparnasse tower the day after the fire. Hide Caption 15 of 47 Photos: In pictures: The fire that devastated Notre Dame Cathedral A woman reacts near Notre Dame the morning after the devastating fire. Hide Caption 16 of 47 Photos: In pictures: The fire that devastated Notre Dame Cathedral A crane lifts experts as they inspect the damage. Hide Caption 17 of 47 Photos: In pictures: The fire that devastated Notre Dame Cathedral People take photos at the scene on April 16, 2019. Hide Caption 18 of 47 Photos: In pictures: The fire that devastated Notre Dame Cathedral People stop to view Notre Dame Cathedral on the morning following the fire. Hide Caption 19 of 47 Photos: In pictures: The fire that devastated Notre Dame Cathedral Fire crews continue to secure the cathedral on April 16, 2019. Hide Caption 20 of 47 Photos: In pictures: The fire that devastated Notre Dame Cathedral Members of the fire department inspect the cathedral. Hide Caption 21 of 47 Photos: In pictures: The fire that devastated Notre Dame Cathedral Notre Dame is seen at sunrise as firefighters continue to work at the cathedral on the morning of April 16. Hide Caption 22 of 47 Photos: In pictures: The fire that devastated Notre Dame Cathedral A handout image photographed on a television screen shows an aerial view of Notre Dame Cathedral engulfed in flames on Monday, April 15, 2019. Hide Caption 23 of 47 Photos: In pictures: The fire that devastated Notre Dame Cathedral French President Emmanuel Macron, center, and his wife Brigitte at the scene of the fire on Monday evening. Hide Caption 24 of 47 Photos: In pictures: The fire that devastated Notre Dame Cathedral Smoke rises in front of the altar cross inside Notre Dame Cathedral. Hide Caption 25 of 47 Photos: In pictures: The fire that devastated Notre Dame Cathedral People pray as Notre Dame Cathedral burns on Monday evening. Hide Caption 26 of 47 Photos: In pictures: The fire that devastated Notre Dame Cathedral A firefighter uses a hose to tackle the flames as the cathedral's roof burns. Hide Caption 27 of 47 Photos: In pictures: The fire that devastated Notre Dame Cathedral Crowds gathered on the banks of the Seine watch the firefighters' progress. Hide Caption 28 of 47 Photos: In pictures: The fire that devastated Notre Dame Cathedral Flames and smoke rise from Notre Dame as a firefighter rushes past. Hide Caption 29 of 47 Photos: In pictures: The fire that devastated Notre Dame Cathedral The fire continues to burn into the night. Hide Caption 30 of 47 Photos: In pictures: The fire that devastated Notre Dame Cathedral The interior of the cathedral roof is seen smoldering. Hide Caption 31 of 47 Photos: In pictures: The fire that devastated Notre Dame Cathedral Located on the Île de la Cité, a small island in the Seine, in central Paris, the cathedral is one of the city's most popular attractions, drawing an estimated 13 million visitors a year. Hide Caption 32 of 47 Photos: In pictures: The fire that devastated Notre Dame Cathedral Smoke and flames rise from Notre Dame cathedral on April 15, 2019. Hide Caption 33 of 47 Photos: In pictures: The fire that devastated Notre Dame Cathedral A woman reacts as she watches flames engulf the roof of the cathedral. Hide Caption 34 of 47 Photos: In pictures: The fire that devastated Notre Dame Cathedral Smoke billows from the cathedral's roof and spire during the fire. Hide Caption 35 of 47 Photos: In pictures: The fire that devastated Notre Dame Cathedral Passers-by watch the cathedral burn. "It's tremendously sad to see this happening to such an iconic monument," bystander Cameron Mitchell told CNN. Hide Caption 36 of 47 Photos: In pictures: The fire that devastated Notre Dame Cathedral Huge clouds of smoke rise into the evening sky as fire engulfs the cathedral. Hide Caption 37 of 47 Photos: In pictures: The fire that devastated Notre Dame Cathedral Firefighters worried the cathedral's two bell towers would be destroyed. Hide Caption 38 of 47 Photos: In pictures: The fire that devastated Notre Dame Cathedral The spire of the landmark cathedral collapses. Hide Caption 39 of 47 Photos: In pictures: The fire that devastated Notre Dame Cathedral The cause of the fire was not immediately known. Hide Caption 40 of 47 Photos: In pictures: The fire that devastated Notre Dame Cathedral A man holds his head in shock as watches the fire take hold. Hide Caption 41 of 47 Photos: In pictures: The fire that devastated Notre Dame Cathedral The roof of the cathedral burns. Hide Caption 42 of 47 Photos: In pictures: The fire that devastated Notre Dame Cathedral Horrified bystanders react to the fire. Hide Caption 43 of 47 Photos: In pictures: The fire that devastated Notre Dame Cathedral A firefighter gets closer to the flames. Hide Caption 44 of 47 Photos: In pictures: The fire that devastated Notre Dame Cathedral Police urged everyone to avoid the area and emergency services directed people away from the scene. Hide Caption 45 of 47 Photos: In pictures: The fire that devastated Notre Dame Cathedral People gather on the banks of the Seine to watch the fire's progress. Hide Caption 46 of 47 Photos: In pictures: The fire that devastated Notre Dame Cathedral Flames roar across the roof of the cathedral. Hide Caption 47 of 47

Patrick Palen, a spokesman for Socra, which was brought in to refurbish statuettes from the cathedral, said workers from the company removed 12 statues of the apostles and four of evangelists from the cathedral last week. Those statues were being held safely in an off-site workshop at the time of the fire. Palen said Socra employees were not at Notre Dame during the fire.

Questions have been raised about why the blaze was not tackled sooner. Security guards at the 850-year-old cathedral first heard the fire alarm at 6:20 p.m. local time on Monday, and evacuated the cathedral even though they didn't see any sign of fire, a spokesman for the Paris Fire Brigade said.

The alarm sounded again some 20 minutes later, at which point the cathedral's security officers noticed the fire, Paris Prosecutor Remy Heitz said.

Fire chiefs denied that they were slow to react to the initial reports of the blaze.

JUST WATCHED Notre Dame: Aerial animation shows the damage caused by fire Replay More Videos ... MUST WATCH Notre Dame: Aerial animation shows the damage caused by fire 00:37

The Crown of Thorns, believed by many to be a relic of the passion of Christ, and the Tunic of Saint Louis were saved. A copper rooster that sat atop the collapsed spire has been recovered intact.

Le Parisien newspaper, citing a source from the Ministry of Culture, said it was not yet known if relics contained in the rooster had survived the blaze.

French billionaire Bernard Arnault, chairman and CEO of luxury good consortium LVMH, pledged $200 million, adding to a purse of $55 million promised by the city of Paris.

Cosmetics company L'Oréal, along with The Bettencourt Meyers family and the Bettencourt Schueller Foundation, has donated €200 million ($226 million) to help fund the repairs.

The Vatican said it will offer technical know-how on rebuilding the cathedral. "We have the type of know-how that the whole world recognizes as being of high quality," Cardinal Gianfranco Ravasi told reporters. "So I think an eventual future offer by the Holy See will be significant."

Prime Minister Edouard Philippe said the contest to replace the spire "will be organized in due course." A committee will be set up to oversee the project and will decide whether the spire should be reconstructed or "adapted to modernity," he said.

Philippe did not say if the rebuild of the cathedral would be finished in time for the 2024 Paris Olympics, but said the work would be done as fast as possible.