Paris prosecutors believe an electrical short-circuit could be the cause of the devastating blaze at Paris' 850-year-old Notre Dame Cathedral.
The cathedral was undergoing renovations when a fire ripped through large parts of the building on Tuesday (NZ time), causing its famous spire to topple.
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Investigators have been questioning staff at the landmark and a source told CNN they were "not excluding any hypothesis at this stage" but an electrical short circuit was the most likely cause.
The fire is still believed to be accidental but other possibilities have not been ruled out.
"While the prosecutor's office does not rule out any hypothesis, we remind that at this stage, nothing in the investigations highlights a criminal origin. Accidental causes remain our privileged lead," the prosecutor's office told CNN.
Investigators still cannot get inside the building for safety reasons. The building is being reinforced with wooden planks to support the structure.
Donations to restore the landmark have now reached more than € 1 billion (NZ$1.6 billion).