PARIS — Only two days after a devastating fire at Notre Dame Cathedral, France on Wednesday launched into a contentious debate over how to restore one of the world’s most treasured architectural wonders.

At issue is whether the French government should seek to re-create the famed cathedral as it was before the blaze, sticking as close as possible to the 850-year-old structure’s pre-fire style and substance. Before Wednesday, many preservationists had assumed that it would.

But Prime Minister Édouard Philippe upended expectations with an announcement of an international competition to replace the iconic spire that collapsed into Monday’s inferno. He also raised the prospect of a 21st-century twist atop a 12th-century creation.

“This is obviously a huge challenge, a historic responsibility,” Philippe said, noting that the new design would have to be “adapted to technologies and challenges of our times.”

He questioned whether “we should re-create” the spire as it was or “as is often the case in the evolution of heritage, we should endow Notre Dame with a new spire.”

Those words were heresy to many traditionalists. “The spire is a masterpiece. It must be rebuilt as it was,” said Benjamin Mouton, Notre Dame’s former chief architect.

“In the coming months we’re going to have a terrible debate,” predicted Stéphane Berhault, an architect who specializes in restorations of historic structures. “It’s always the same in France. Everyone will want to impose their point of view.”

That debate was ramping up early because French President Emmanuel Macron has plowed ahead with an ambitious timeline for the building’s reconstruction.

Experts predict the process will take a decade or more, with many warning that nasty surprises lurk as engineers test the stability of the structure’s exterior stones. As of Wednesday, the charred cathedral was still considered so hazardous that investigators seeking to pinpoint the cause of the blaze were forbidden from entering.

But Macron told the nation he wants the work to be done within five years, a timeline that would have the building ready by the time Paris hosts the 2024 Summer Olympics.

That’s a breakneck pace for a project of monumental scale that will involve hundreds, if not thousands, of master craftsmen, carpenters, masons and others working inch by painstaking inch to restore the structure to its past glory.

Or perhaps, most daringly of all, to reimagine it.

French President Emmanuel Macron spoke "directly" about the disastrous fire at Notre Dame Cathedral April 16, vowing to "rebuild Notre Dame Cathedral." (Reuters)

Macron hinted in his address Tuesday night that he wants to turn tragedy into opportunity, saying he believes the structure can be rebuilt “even more beautiful” than before.

Philippe’s announcement of the design competition offered confirmation that French authorities are interested in putting their own stamp on a building that began with the laying of the cornerstone in 1163, as King Louis VII looked on, and has been continuously revised in the centuries since.

In some quarters Wednesday, that news was welcomed.

“France wants to show the will to open discussion, the will to innovate,” said Gosia Kotula, an architect.

Kotula has a special degree and designation — known in France as an “architecte du patrimoine” — that allows her to work on the restoration of historic buildings.

In France, where buildings are not truly old unless they date to the Middle Ages or before, that is a cottage industry, with cathedrals, palaces, concert halls and more forever in need of rehabilitation to keep at bay the ravages of time.

The dominant approach to restoration among today’s architects, Kotula said, is to preserve buildings and features as faithfully as possible without interjecting modern elements.

But she noted that Notre Dame is full of touches and flourishes that reinterpreted and remade the original Medieval structure for a new era — the spire, for instance.

Until Monday, it may have been impossible for visitors to imagine the cathedral without the nearly 300-foot adornment, which was made of wood, sheathed in lead and pointed to the heavens.

But that particular spire was added in the mid-19th century, when Eugène Viollet-le-Duc — the leading restoration architect of his day — decided to give the already awe-inspiring cathedral a new, updated exclamation point. An earlier spire had been removed in the late 1700s after centuries of wind damage.

“We think it’s a creation of Medieval times,” Kotula said. “It’s not.”

The spire is hardly the only element likely to be contested as officials work their way toward a master plan for a restoration that is as complex as any that French experts say they have seen.

Other French cathedrals — in Nantes, or in Rennes — have burned and been restored. But “there’s never been a project more important than this,” said Christophe Villemain, who operates a construction business that has carried out restorations of historic buildings across the nation.

The blaze cost Notre Dame two-thirds of its roof and could have brought the entire structure crashing down had firefighters not contained it when they did.

Paris prosecutors said Wednesday that they continue to believe the fire was accidental. But the exact cause remains a mystery.

In photos: Paris’s Notre Dame Cathedral on fire

April 16, 2019 | Firefighters carry out inspections on the damaged roof of Notre Dame Cathedral in Paris. (Dan Kitwood/Getty Images)

Investigators are interviewing dozens of workers who had been involved in a renovation project at the cathedral. Although none of the workers are believed to have been present when the fire broke out Monday evening, police hope they can offer clues to what sparked the blaze.

Le Parisien reported on one theory, that an electrical fault in a temporary elevator may have been to blame. But the news outlet quoted a person close to the investigation as saying that the probe would be “long and difficult” and that “it is possible that we never know what is behind this fire.”

Whatever the cause, the scale of destruction was formidable — and not yet fully known.

Experts said that just assessing the damage will take a year or more and that the stone exterior — saved from collapse Monday — may still need to be significantly repaired or replaced.

The structures supporting the cathedral’s stained-glass rose windows are especially vulnerable and will be partially dismantled, the Culture Ministry’s fire expert, Jos Vaz de Matos, said Wednesday.

Whatever the ultimate scale of the work, planners will have a considerable budget. A fund set up to aid the renovation hit 1 billion euros Wednesday, or more than $1.1 billion.

Among the most immediate challenges in the restoration is the roof. A temporary one will need to be installed to prevent further damage to the interior. Then there will be the question of whether the permanent replacement should seek to replicate the original.

The roof support structure was known as “the forest,” and each timber was hewed from an ancient oak tree. The actual forest where the trees were harvested is long gone, and many have noted this week that trees as old and tall as that are now uncommon. But planners could still rebuild with oak that closely approximates the original wood.

“The challenge of the restoration is to make Notre Dame beautiful and solid and to do it as it was done by architects during the Middle Ages,” said Berhault, the architect.

Not everyone agrees that is the overriding goal.

Paris firefighters chief warrant officer Philippe Demay described the fire at Notre Dame as "a catastrophe," but his crew managed to save its iconic towers. (AP)

For a design element that few will ever see — the wood beams support a metal roof — there is no reason not to choose a lighter and less expensive replacement, said Félix Bulcourt of Paris’s ENS design school.

Measures to improve fire safety, he said, should also be implemented in a building that withstood centuries without a serious blaze but would never meet today’s rigorous standards.

The result may not be a replica of the original. But that, Bulcourt said, is an impossible goal. “The question is: Do we want something static? Or do we want a living building?”

Christophe Chabert and Quentin Aries contributed to this report.