Officials attending the Notre Dame blaze initially couldn't find any sign of fire despite the alarm being raised 23 minutes before the structure went up in flames.
Paris public prosecutor Rémy Heitz announced on Tuesday that firefighters were called to the world-famous cathedral at 6.20pm CET, almost half an hour before the roof caught fire after a fire alarm was triggered.
But after an initial inspection they said they were unable to find any evidence of fire.
Firefighters remained inside Notre Dame until a second alarm went off at 6.43pm and eventually discovered a fire in the network of wooden beams under the cathedral's roof - but were unable to put it out.
Mr Heitz said this evening: 'What we know at this stage is that there was an initial alarm at 6:20 p.m., followed by a procedure to verify this but no fire as found.
'Then, there was a second alarm at 6:43 p.m. and at that point a fire was detected in the structure.
'The investigation is going to be long and complex,' he added. 'We are in the process of interviewing witnesses.'