Experts now believe the inferno was started by a discarded cigarette, that could have begun carbonising tinder-dry wood without anyone noticing. Forensic experts told a criminal inquiry into Monday's blaze this was a likely scenario. A source close to the enquiry said: "This is very common in the old buildings around central Paris. "There would not have been much energy in the fire to begin with, and certainly not enough to trigger alarms. This would have changed as the combustion intensified and sparked a full-blown fire on the roof of the building."
Specialist restorers had been working on scaffolding surrounding the wood and lead roof and spire.
But workers from the Bras Brothers, a company based in Jarny, north-east France, said they were not on site when the fire erupted into a full-blown inferno soon after 6pm on Monday.
The source added: "Most of the restorers have been interviewed but deny being in the cathedral when the fire started.
"They say the fire alarms around the scaffolding itself did not go off at all.
"The likelihood of a slow burning fire changes everything." Julien Le Bras, chief executive of the Bras Brothers, has offered "full assistance" to the inquiry being led by Paris prosecutors. Mr Le Bras has insisted all proper safety procedures were followed.
