French detectives investigating the Notre Dame fire questioned workers who had been restoring the magnificent Gothic cathedral and left the site minutes before the blaze broke out.
The inferno that consumed most of the roof was probably started accidentally, said Rémy Heitz, the Paris prosecutor, possibly as a result of restoration work on crumbling upper sections of the cathedral and the spire.
"Nothing indicates a deliberate act," he said.
Some 50 investigators from the Paris police criminal division — half of its detectives — are assigned to the case.
Among those they have interviewed are 12 employees of the firm that was leading the €150 million four-year restoration, Le Bras Brothers, based in Jarny, north-eastern France.
The highly respected specialist company has restored cathedrals and churches in Strasbourg, Reims, Poitiers, Verdun, Amiens and other French cities.
It had recently started restoring the 90m wooden spire added to the 850-year-old cathedral in the 19th century, which collapsed in flames.
"All I can tell you is that at the moment the fire began none of my employees were on the site. We respected all procedures," Julien Le Bras, a representative of family firm Le Bras Freres.
Another company, Europe Echafaudage, erected 250 tonnes of scaffolding and a lift.
"Detectives have been talking to some of the restoration staff to see if they can help with pinpointing the exact location where the fire started, probably in the roof," a source close to the investigation said.
"They're also looking at what equipment was being used, especially power sources and lighting that may have contributed to starting a fire."
Le Bras, the head of Bras Brothers, was brimming with pride when he gave an interview in 2017 about his firm being chosen to restore Notre Dame.
"No one has touched it in 150 years, it's really quite something to say we will be the ones getting involved. We won't be offering a guarantee of 10 years, but 150 years," he said.
When a French reporter called Le Bras on the night of the fire to ask what was happening, he simply repeated in a panicked voice "I don't know, I don't know," before hanging up.
Today he said none of his workers was present when the fire started.
Bernard Fonquerie, an architect who worked on Notre Dame from 1978 to 1990, said: "The inquiry will show what happened."
"A building like this has the makings of a disaster. You could never build a cathedral like this today - the rules have completely changed," he said.
"In the years that I worked here, we never had an accident because we followed the rules so tightly. There are extinguishers everywhere, you may not use flammable liquids."
Experts said bureaucracy and politics had delayed the much-needed restoration of Europe's most visited monument, parts of which had been left in a state of disrepair.
Marie-Claude Gauvard, a historian and author of a book on Notre-Dame said: "The resources needed to maintain it were not provided. "When the renovation was finally started, it was high time, and perhaps a bit late."
Until last year, no major restoration had been carried out since the mid-19th century, after Victor Hugo called attention to the need to repair damage inflicted during the French revolution.