Saving France's 850-year-old Notre-Dame cathedral came down to a crucial time window of 15-30 minutes, France's deputy interior minister has said.
Laurent Nuñez praised the "courage and determination" of firefighters who "risked their own lives" to salvage the building's stone structure and its two towers.
The fire ravaged the cathedral's roof and caused its spire to collapse.
French President Emmanuel Macron has vowed to rebuild it within five years.
The cause of the blaze is unclear.
"We now know it all came down to 15-30 minutes," Mr Nuñez said, adding that police and fire services would spend the next 48 hours assessing the security and safety of the structure.
Paris public prosecutor Rémy Heitz said his office was "favouring the theory of an accident", but had assigned 50 people to investigate the origin of the fire.
Other officials have suggested it could be linked to extensive renovation works taking place at the cathedral.
Thoughts are now turning to how Notre-Dame will be rebuilt, which Mr Macron promised to make "even more beautiful".
"We will turn this catastrophe into an opportunity to come together", he said.
In a televised public address, Mr Macron also heaped praised on the fire services.
"The firefighters stopped the fire by taking the most extreme risks. They were 20 or 25, from each corner of France, from each region."
A number of companies and business tycoons have so far pledged about €700m ($913m; £606m) between them to help with reconstruction efforts, Le Monde newspaper reports.
Offers of help have also poured in from around the globe, with European Council President Donald Tusk calling on EU member states to rally round.
Eric Fischer, head of the foundation in charge of restoring the 1,000-year-old Strasbourg cathedral, told AFP the Notre-Dame may take "decades" to rebuild.
The blaze was discovered at 18:43 local time (16:43 GMT) on Monday, and firefighters were called. The flames quickly reached the roof of the cathedral, destroying the wooden interior before toppling the spire.
Fears grew that the cathedral's famous towers would also be destroyed.
But while a number of fires did begin in the towers, Mr Nuñez said they were successfully stopped before they could spread.
By the early hours of Tuesday, the fire was declared under control, with the Paris fire service saying it was fully extinguished by 10:00 local time (08:00 GMT).