The fire at Notre Dame Cathedral on April 15, 2019, is believed to have started accidentally, with investigators considering two main possible causes:

1. A cigarette that was poorly extinguished by workers on the scaffolding used for restoration work. Some workers admitted to smoking on the scaffolding, and cigarette butts were found beneath it. However, there was no evidence that smoking occurred inside the cathedral itself.
2. An electrical short circuit possibly caused by damage to the wiring related to the cathedral's electric bells, which were still operational during the restoration and had cables running through the roof area. The bells ringing at 18:04 preceded the fire being declared at 18:20.

The fire began in the attic beneath the cathedral's roof, an area called "la forêt" due to its wooden lattice structure. It was intensified by the absence of a sprinkler system and other fire safety measures. The fire detection system was somewhat flawed, leading to delayed recognition and response. The exact cause has not been definitively established, leaving open the possibility of either a cigarette or electrical issue, or a combination of safety negligence and unfortunate circumstances during the renovation works.