Experts from three police agencies will take samples and search for clues as to what caused the blaze, a police official told The Associated Press on condition of anonymity because he wasn't authorized to be identified in media reports.
Police made a preliminary visit last week but weren't able to conduct a thorough examination until the cathedral's structure was secured. Teams in white helmets talked with firefighters outside the monument Thursday, while workers shored up the roof and sides of the 12th-century cathedral. Multiple cranes are positioned around the site, as the French president pushes for a speedy reconstruction after the fire consumed the cathedral's roof and knocked down its spire.
Officials believe the fire was accidental, possibly caused by a short circuit linked to renovation work underway before the fire. Workers involved in that renovation told investigators that they had smoked on the construction site and left behind cigarette butts, according to the spokesman Marc Eskenazi of Le Bras Freres construction companies.
"It's not good. We condemn this," Eskenazi told the AP. But he insisted that "a cigarette butt did not burn down Notre Dame. It's just not possible." He cited investigators as saying the fire started inside the cathedral, while the workers were only on the roof. He said the last worker left the building at 5:50 p.m. the night of the fire, while the first fire alarm went off a half-hour later.
He said the worker switched off the electricity to the elevators and the lighting used for the renovation work before leaving. The Paris prosecutor's office wouldn't comment on specifics of the investigation, but said all potential leads are being pursued. The prosecutor's office and Paris police wouldn't comment on a report in investigative weekly Le Canard Enchaine that electric cables to the cathedral's bells had remained activated despite the renovation work.
While the cathedral will remain closed to the public for several years, tourists massed in the neighborhood Thursday to get a closer look at the damage and the work on the building's facade. France's government presented a bill Wednesday aimed at speeding up the reconstruction of Notre Dame Cathedral and providing transparency on how huge amounts of donation money for the restoration will be spent.
President Emmanuel Macron has set a goal of rebuilding the cathedral in just five years —which some experts consider simply impossible to reach.