Investigators think an electrical short-circuit was the most likely cause of the Notre Dame Cathedral fire, according to a French judicial police official.

The official said investigators still do not have the green light to work in the cathedral and search in the rubble for safety reasons.

He said the monument is still being consolidated with wooden planks to support some fragile parts of the walls.

The probe was taking place as France paid a day-long tribute to the Paris firefighters who saved the famous cathedral from collapse and rescued its treasures.

The tributes came as construction workers rushed to secure an area above one of the church's famed rose-shaped windows and other vulnerable sections of the fire-damaged landmark.

Notre Dame fire: Global tributes Show all 5 1 /5 Notre Dame fire: Global tributes Notre Dame fire: Global tributes Fenice theatre, Venice The Fenice theatre in Venice illuminated with the colors of the French flag EPA Notre Dame fire: Global tributes The dome of the Chapel of the Shroud, Turin The dome of the Chapel of the Shroud in Turin illuminated with the colors of the French flag EPA Notre Dame fire: Global tributes One World Trade Centre, New York City One World Trade Centre is lit in the colors of the French flag in solidarity with the people of France REUTERS Notre Dame fire: Global tributes Stari Most bridge, Mostar The Stari Most bridge in the Bosnia and Herzegovina city of Mostar lit in colours of the French flag AFP/Getty Images Notre Dame fire: Global tributes Empire State Building, New York City Empire State Building is lit in the colors of the French flag in solidarity with the people of France REUTERS

President Emmanuel Macron held a ceremony at the Elysee Palace to thank the hundreds of firefighters who battled the fast-moving fire at Notre Dame for nine hours, preventing the structure's destruction and rescuing many of the important relics held inside.

"We've seen before our eyes the right things perfectly organised in a few moments, with responsibility, courage, solidarity and a meticulous organisation", Mr Macron said. "The worst has been avoided."

Mr Macron said the firefighters will receive a medal for their courage and devotion.

The huge cathedral, including the spire that was consumed by flames and collapsed, was in the initial stages of a lengthy restoration.

The roof was destroyed, but Notre Dame's famous bell towers, rose windows, organ, and precious artworks were saved.

Fire officials warned the building remains very fragile and extremely dangerous for construction workers, restoration experts and neighbours.

Notre Dame fire: What was lost and what was saved Show all 13 1 /13 Notre Dame fire: What was lost and what was saved Notre Dame fire: What was lost and what was saved Statue of Saint Denis Notre Dame https://commons.wikimedia.org/wiki/File:Paris_-_Cath%C3%A9drale_Notre-Dame_-_Portail_de_la_Vierge_-_PA00086250_-_003.jpg Thesupermat/CC-BY-SA Thesupermat/CC-BY-SA Thesupermat/CC-BY-SA Notre Dame fire: What was lost and what was saved "Close up of Virgin Mary and Baby Jesus statues at Notre Dame Cathedral in Paris, France. Other images of the Notre Dame Cathedral:" gregobagel Getty Images/iStockphoto Notre Dame fire: What was lost and what was saved Safe: Rose windows Among the most famous architectural features of the Gothic masterpiece, the stained glass rose windows are treasured artworks. The three rose windows, which date back to the 13th century, adorn the north, south and west facades. There were hopes the windows had escaped being destroyed by the fire after firefighters stopped its spread. Photos the following morning suggested the circular window of the nave had remained intact. AFP/Getty Notre Dame fire: What was lost and what was saved Destroyed: Irreplaceable wooden roof Crowds of tourists and Parisians watched in horror as the spire and roof of Notre Dame cathedral came crashing down to the ground. EPA Notre Dame fire: What was lost and what was saved Safe: Descent from the Cross The cross and statue on Notre Dame's high altar - sculpted by Nicolas Coustou in 1723 - were believed to have survived in tact despite being surrounded by smoke and debris. Reuters Notre Dame fire: What was lost and what was saved Safe: Great Organ With nearly 8,000 pipes, some dating back to the 1700s, Notre Dame's master organ is one of the largest in the world. The monumental instrument, the largest in France, was fully restored in 2013 with each pipe cleaned. Paris' deputy mayor, Emmanuel Gregoire, said the instrument remained intact following the fire. AFP/Getty Notre Dame fire: What was lost and what was saved Safe: Bells Housed in the two western towers, Notre Dame's bells have rung out at key moments in France's history. Emmanuel, the largest bell, was lifted into the south tower in 1685 and weighs over 23 tonnes. The fire was prevented from spreading to the bell towers. AFP/Getty Notre Dame fire: What was lost and what was saved Safe: Crown of Thorns Believed to be a relic of the wreath of thorns placed on the head of Jesus Christ at his crucifixion, the object was stored in the cathedral's treasury. French King Louis IX brought the relic, which is contained in an elaborate gold case, to Paris in 1238. The Crown of Thorns was saved and has been taken into safekeeping. AP Notre Dame fire: What was lost and what was saved Safe: Tunic of Saint Louis The tunic allegedly worn by Saint Louis IX as he brought the Crown of Thorns to Paris was kept inside the cathedral. It has been saved. AFP/Getty Notre Dame fire: What was lost and what was saved Safe: Statues on the roof The week before the fire, religious statues set atop the cathedral were removed for the first time in over 100 years as part of the restoration project. A 100-metre-high crane lowered the copper statues representing the 12 apostles and four evangelists onto a lorry. AFP Notre Dame fire: What was lost and what was saved Destroyed: Spire Crowds of tourists and Parisians watched in horror as the spire and roof of Notre Dame cathedral came crashing down to the ground. EPA/I Notre Dame fire: What was lost and what was saved Unknown: Choir screen It is not known what happened to the detailed choir screen at Notre Dame cathedral. AFP/Getty Notre Dame fire: What was lost and what was saved Some of the artworks from Notre-Dame sheltered as the fire devastated the cathedral AFP/Getty

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

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 in the devastating blaze.

They were also securing the support structure above one of Notre Dame's rose windows with wooden planks.

Notre Dame before and after Show all 19 1 /19 Notre Dame before and after Notre Dame before and after The cathedral with the spire standing tall (left) and (right) after the fire AFP/Getty Notre Dame before and after As viewed from the Montparnasse Tower before the fire AFP/Getty Notre Dame before and after As viewed from the Montparnasse Tower after the fire AP Notre Dame before and after The heart and transept before and one day after the fire AFP/Getty Notre Dame before and after The nave of the cathedral before the fire Alamy Notre Dame before and after The nave of the cathedral after the fire Reuters Notre Dame before and after The rose window in the cathedral before the fire AFP/Getty Notre Dame before and after After the fire a hole has been exposed in the roof above the window Twitter Notre Dame before and after The cathedral from the Seine before and after Reuters Notre Dame before and after Inside the cathedral before the fire Reuters Notre Dame before and after The burning roof has left a hole in the nave Reuters Notre Dame before and after The nave after the fire was extinguished AP Notre Dame before and after Inside the cathedral after the fire Reuters Notre Dame before and after A view from the Seine of the cathedral before and after the fire Reuters Notre Dame before and after An aerial shot of Notre Dame before the fire AFP/Getty Notre Dame before and after An aerial shot of Notre Dame during the fire shows the wooden roof burning and collapsing AFP/Getty Notre Dame before and after Tourists gather near the cathedral and then seen afterwards Reuters Notre Dame before and after The spire of the cathedral is shown before and during the fire AFP/Getty Notre Dame before and after A scorched section of the exterior after the fire Reuters

Remarkably, no one was killed in the fire, which began during a Mass, after firefighters and church officials speedily evacuated those inside.

Among the firefighters honoured was Paris fire brigade chaplain Jean-Marc Fournier, who said he was falsely credited with helping salvage the crown of thorns believed to have been worn by Jesus at his crucifixion.

The chaplain said a team of rescuers broke the relic's protective covering and an official who had the secret code to unlock the protection finished the job.

Newspapers react to the Notre Dame fire Show all 14 1 /14 Newspapers react to the Notre Dame fire Newspapers react to the Notre Dame fire newspaper-front.jpg Newspapers react to the Notre Dame fire notre-dame-fronts-9.jpg Newspapers react to the Notre Dame fire notre-dame-fronts-10.jpg Newspapers react to the Notre Dame fire notre-dame-fronts-3.jpg Newspapers react to the Notre Dame fire notre-dame-fronts-5.jpg Newspapers react to the Notre Dame fire notre-dame-fronts-13.jpg This photo illustration taken in Madrid on April 16, 2019 shows Spanish daily newspapers with front page coverage of the Notre-Dame Cathedral fire in Paris. - The raging fire that tore through Notre-Dame Cathedral in central Paris was finally extinguished on April 16 after the blaze brought its towering spire and roof crashing to the ground, wiping out centuries of priceless heritage central to French culture and history. (Photo by GABRIEL BOUYS / AFP)GABRIEL BOUYS/AFP/Getty Images AFP/Getty Newspapers react to the Notre Dame fire notre-dame-fronts-1.jpg Newspapers react to the Notre Dame fire notre-dame-fronts-2.jpg Newspapers react to the Notre Dame fire notre-dame-fronts-7.jpg Newspapers react to the Notre Dame fire notre-dame-fronts-4.jpg Newspapers react to the Notre Dame fire notre-dame-fronts-14.jpg Israeli and Palestinian newspapers, featuring images of the fire that ravaged the Parisian Notre Dame cathedral on their front pages, are are pictured in Jerusalem, on April 16, 2019. - Paris was struck in its very heart as flames devoured the roof of Notre-Dame, the medieval cathedral made famous by Victor Hugo, its two massive towers flanked with gargoyles instantly recognisable even by people who have never visited the city. (Photo by THOMAS COEX / AFP)THOMAS COEX/AFP/Getty Images AFP/Getty Newspapers react to the Notre Dame fire notre-dame-fronts-8.jpg Newspapers react to the Notre Dame fire notre-dame-fronts-6.jpg Newspapers react to the Notre Dame fire notre-dame-fronts-12.jpg Lebanese newspapers, featuring images of the fire that ravaged the Parisian Notre Dame cathedral on their front pages, are spread out at a newspaper satll in the capital Beirut, on April 16, 2019. - Paris was struck in its very heart as flames devoured the roof of Notre-Dame, the medieval cathedral made famous by Victor Hugo, its two massive towers flanked with gargoyles instantly recognisable even by people who have never visited the city. (Photo by ANWAR AMRO / AFP)ANWAR AMRO/AFP/Getty Images AFP/Getty

Among others honoured was Myriam Chudzinski, one of the first firefighters to reach the roof as the blaze raged.

Loaded with gear, they climbed hundreds of steps up the cathedral's narrow spiral staircase to the top of one of the two towers. She had trained at the site for hours for just this moment.

"We knew that the roof was burning, but we didn't really know the intensity," she told reporters. "It was from upstairs that you understood that it was really dramatic. It was very hot and we had to retreat, retreat. It was spreading quickly."

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.

The building would have burned to the ground in a "chain-reaction collapse" had firefighters not moved as rapidly as they did to battle the blaze racing through the building, said Jose Vaz de Matos, a fire expert with France's Culture Ministry.

Mr Macron wants to rebuild the cathedral within five years - in time for the 2024 summer Olympics that Paris is hosting - but experts say the vast scale of the work to be done could easily take 15 years, since it will take months, even years, just to figure out what should be done.