Many priceless items were saved from Notre Dame Cathedral on Monday after a massive fire tore through the structure. But the 12th century Parisian landmark itself has a long — and expensive — road ahead to full restoration.

Another half-hour and the blaze could have entirely collapsed the cathedral, French authorities told The Guardian. It left behind damages the scale of which is still unknown. The restoration project could take years and likely millions more than the funds donated thus far — and is unlikely to unfold without controversy, experts say.

Pledges toward the restoration efforts surpassed 700 million euros, or $790 million, in the first few days. But the full extent and cost of the damages remains to be seen.

Investigators haven't been able to enter the cathedral to assess what might have caused the fire. The Paris public prosecutor's office has stated that it appears to have been an accident, and that it has no reason to believe it was arson.

The first fire alert was sounded at 6:20 p.m. local time on Monday evening. The inferno brought as many as 400 firefighters to the scene and it was nearly 15 hours before they were able to put out the flames.

Before restoration can begin, a team of architects, engineers and preservation specialists will likely go in and assess what is there, and what needs to be replaced — stone by stone — said Kevin Murphy, a professor of humanities at Vanderbilt University.

Structural engineers will have to assess the strength of the building and scaffolding will be set up on the inside and outside of the church to ensure that it is safe for experts to go in, according to Krupali Krusche, an associate dean at the University of Notre Dame School of Architecture and expert on historical preservation.

"The assessment part takes some time — at least a year or a year and a half of assessing the project," Krusche said.

Krusche worked on the reconstruction of the Dresden Frauenkirche, a church in Germany that was reconstructed between 1998 and 2006 after being bombed during World War II.

"Nothing remained. The whole church was completely reconstructed. The heat levels were 1,000 degrees so mostly everything burned down," Krusche said. According to Krusche, that project cost 118 million euros, or $133 million.

"I would presume just because of the scale of [Notre Dame], this would be at least three to four times that," she said. "This depends on the quality of the reconstruction. There are preservation techniques that allow you to get the aesthetic look but are not as expensive. The more authentic, in terms of the materials, you get the more expensive things get."

The nearly $800 million pledged so far would provide for a good reconstruction, but an expert restoration would likely require more, Krusche said. She estimated costs could balloon to $1 billion.

There will also be much to decide in terms of restorations to be carried out on the building including which of the 19th century additions, such as the spire that was destroyed in the fire, will be replicated or replaced. Renovators will be consulting with scholars, architects and church officials in order to determine how to proceed, Murphy said.

Krusche said the consensus among experts is usually to return the building to the way it looked the day before the fire. "There will be older and newer stones with different color and that's the historical contribution of this event," she said.

Then there are the costs to move and restore items held within the cathedral.

Precious items including the Crown of Thorns, believed to have been placed onto Jesus' head during the crucifixion; Le Grand Orgue, an instrument which dates back to the 1730s; and the Tunic of Saint Louis, a long garment dating to the 13th century, were all spared from the flames.

These items, along with other treasures, are now being held at the Paris City Hall, said French Culture Minister Franck Riester on Tuesday outside of the cathedral. Sixteen statues of saints were removed from the cathedral last Thursday for cleaning and therefore escaped the fire.

The rest of the treasures will be stored in the Louvre Museum as soon as they can be retrieved, Riester added. The large paintings being stored in the church suffered smoke damage, he said, and will be removed from Notre Dame on Friday morning. They will then be conserved and restored.

Not everything that was lost can be restored to exactly the way it was. France will launch an international architectural competition to design a new spire for the cathedral, Prime Minister Edouard Philippe announced Wednesday. Most notably, the roof of the cathedral that was made of wood went up in flames and suffered severe damages.

"The timber that was used [in the roof] was cut down hundreds of years ago from forests that don't exist anymore in the quantity they would need for that huge of a structure," Murphy highlighted. "It would be difficult if not impossible to obtain timber on the scale that was in the roof."

On Tuesday French President Emmanuel Macron vowed to complete restoration within the next five years. Krusche thought that was feasible, assuming a team of experts as large as 100 people.

Murphy isn't as convinced.

"If it were to be renovated in 10 years that would be ambitious," he said.

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