Paris police investigators think an electrical short-circuit most likely caused the fire at Notre-Dame Cathedral, a police official said on April 18 , as France paid a daylong tribute to the firefighters who saved the world-renowned landmark.

A judicial police official told The Associated Press that investigators made an initial assessment of the cathedral Wednesday but don’t have a green light to search Notre on Dame’s charred interior because of ongoing safety hazards.

The cathedral’s fragile walls were being shored up with wooden planks, said the official, who spoke on condition of anonymity because he wasn’t authorized to speak by name about an ongoing investigation.

Investigators so far believe the fire was accidental, and are questioning both cathedral staff and workers who were carrying out renovations. Some 40 people had been questioned by, according to the Paris prosecutor’s office.