The contrast is striking. The cathedral reopened its doors in December 2024, five years and eight months after the fire. The reconstruction, which had been described as almost impossible to complete within such a short timeframe, has become a spectacular demonstration of French expertise. And yet, in the legal arena, time seems to have stood still since the evening of the fire. Seven years on, the exact cause of the fire has still not been definitively made public. Reports as recent as April 2026 indicated that the courts had not publicly determined the precise cause of the blaze.
We must go back to the beginning. On 15 April 2019, shortly after 6.30 pm, a fire broke out in the upper parts of the cathedral. The medieval roof structure went up in flames, the roof was ravaged and Viollet-le-Duc’s spire collapsed before the eyes of television cameras from around the world. Within hours, the Paris Public Prosecutor’s Office launched an investigation. On 26 June 2019, a judicial inquiry against persons unknown was opened on the grounds of ‘unintentional damage by fire resulting from a manifestly deliberate breach of a duty of care or safety’. Three investigating judges were then assigned to the case.
Very quickly, the criminal line of inquiry lost ground. After hundreds of interviews, the analysis of images, technical documents and findings from the scene, the Public Prosecutor’s Office stated that there was no evidence to support the hypothesis of arson. This point is crucial: the fact that the cause remains unknown does not mean that all hypotheses are equally plausible. J Up until the latest known statements from the public prosecutor’s office, nothing had pointed the investigation towards a deliberate or terrorist act.
That leaves an accident. But which one?
Two main hypotheses have been considered in the case since 2019. One is that of a cigarette butt that was not properly extinguished, in an area where some workers had admitted to smoking despite the ban, and the other is that of an electrical fault. Investigators have focused in particular on the numerous cables in the attic and on electrified bells installed several years before the fire. Other technical possibilities have been examined in relation to the restoration work then underway. However, none of these lines of inquiry has, publicly, provided evidence sufficient to state: this is how Notre-Dame burned down.
The investigation is extraordinarily complex for a fairly obvious reason: the fire destroyed the very scene where it started. The roof structure where the flames first broke out has largely disappeared. The materials burnt, fell, were crushed and then became mixed up with the rubble. Furthermore, the molten lead and the risks to the building’s stability have considerably complicated the investigators’ work. Tons of rubble were collected and analysed right up until 2022. An area considered particularly important for pinpointing the source of the fire remained cordoned off for a long time. As late as March 2024, judges, criminal investigation officers and experts returned to Notre-Dame.
In the autumn of 2024, however, a significant breakthrough was announced: the analyses of samples taken from the rubble had been completed. A summary of the expert reports was expected to help pinpoint the time and, above all, the exact location where the fire started. According to information published at the time, the fire may have started at a horizontal beam, a ‘sabblière’, in the south-eastern section of the transept crossing. But pinpointing the source of the fire is not the same as explaining what caused it.
This is where one of the most fascinating stages of the investigation comes in: a **three-dimensional model of the fire**. Investigators and experts have access to an exceptional amount of photographs and videos taken by tourists, local residents, journalists and security cameras in the vicinity of Notre-Dame on 15 April 2019. The principle involves digitally reconstructing the flames and their progression in order to trace them back, as far as possible, to their point of origin and compare this trajectory with various technical hypotheses. In late 2024 and again in April 2025, the public prosecutor’s office explained that this analysis would require several further months.
And no one had been placed under investigation at that stage. This is another important aspect of the case. Six years after the fire, despite a thousand pages of case files from the very early stages of the investigation, hundreds of interviews, 96 items seized and numerous expert reports, no individual had been charged with a criminal offence.
This is precisely what makes the story almost mind-boggling. To rebuild Notre-Dame, it was necessary to understand how centuries-old beams were assembled, to reproduce Viollet-le-Duc’s spire, to map the cathedral using billions of digital points, and to coordinate tradespeople from all over France. Laser scans, photogrammetry, drones and digital models of spectacular precision have made it possible to bring back to life what had been lost.
Technology has therefore succeeded in reconstructing the past. It is still struggling to reconstruct the first few seconds of the fire.
We must, of course, resist the temptation to believe in conspiracy theories. The fact that an investigation is dragging on does not prove that something is being concealed. Rumours have also claimed that the case was closed without further action, dismissed or even classified as ‘secret-defence ’. The Paris Public Prosecutor’s Office had formally denied these claims in December 2024: the judicial investigation was still ongoing.
But public scrutiny remains entirely legitimate. Notre-Dame is not just any building. The fire caused a global outcry, triggered an exceptional financial and human response, and gave rise to one of the most significant contemporary heritage restoration projects. It is therefore only natural to want to understand not only how it was saved and rebuilt, but why it nearly disappeared.
France has kept an extraordinary promise: to return Notre-Dame to the French people and to the world. It has managed, in just a few years, to rebuild what generations of builders had taken decades to construct. Now there remains another promise, much more discreet and yet essential: that of judicial truth.
Seven years on from 15 April 2019, Notre-Dame stands once more. But the burning question that arose the very day after the fire has still not been fully answered: who or what sparked the first flame?
