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

A French 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 Dame’s charred interior because of ongoing safety hazards.

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

Earlier in the afternoon, French President Emmanuel Macron held a ceremony at the Elysee Palace to thank the hundreds of firefighters who battled the fast-moving fire for nine hours starting Monday evening. Authorities have said Notre Dame, which dates from the 12th century, was in danger of going up in flames before fire crews stopped it from spreading into a tower belfry.

Fire responders also rescued many of the important relics and works of art inside the cathedral.

“We’ve seen before our eyes the right things perfectly organized in a few moments, with responsibility, courage, solidarity and a meticulous organization,” Macron said. “The worst has been avoided.”

Macron said the firefighters will receive an Honor Medal for their courage and devotion.

As the ceremony took place, investigators continued seeking clues to what sparked the fire. The huge cathedral, including the spire that was consumed by flames and collapsed, was in the initial stages of a lengthy restoration.

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 Thursday, according to the Paris prosecutor’s office.

Fire officials warned that the building remains unstable and extremely dangerous, including for the construction workers who rushed to secure an area above one of the rose-shaped windows and other vulnerable sections of the fire-damaged structure.

Workers using a crane were removing some statues to lessen the weight on the cathedral’s fragile gables, or support walls, and to keep them from falling, since the section lacked the support of the massive timber roof that burned up in the devastating blaze.

Police, citing “important risks” of collapse and falling objects, officially closed Thursday a large swath of the island in the Seine River on which Notre Dame sits. The area had been unofficially blocked off since the fire.

Paris City Hall also was holding a ceremony in the firefighters’ honor Monday afternoon, with a Bach violin concert, two giant banners strung from the monumental city headquarters and readings from Victor Hugo’s The Hunchback of Notre Dame.

Nicolas Vaux-Montagny and Angela Charlton in Paris contributed.