A Paris judicial official says investigators have questioned about 30 people after the fire at Notre Dame Cathedral.
He said most of them were employees working on the renovation of the monument.
The official, speaking anonymously on an ongoing investigation, said the cathedral's fire alarms sounded twice on Monday evening.
The first time, some people, including a fire official permanently working on the site, went to check under the roof and saw nothing. The second time it was already too late because the fire was too strong, the official said.