PARIS (NYTIMES, REUTERS, WASHINGTON POST) - French investigators have already started questioning witnesses for possible clues on the cause of the fire that badly damaged Paris's iconic Notre-Dame Cathedral.

Fifty people are working on what would be a long and complex investigation, officials said.

Paris public prosecutor Remy Heitz said there was no obvious indication the fire was arson. The fire swiftly ripped through the cathedral's oak roof supports, where workmen had been carrying out extensive renovations to the spire's timber-framed supports.

The Paris prosecutor's office said late on Tuesday (April 16) that investigators had questioned about 30 witnesses. These included workers from the companies involved in the restoration of the cathedral not long before the fire broke out, and staff at Notre-Dame in charge of security.

Five companies were doing renovation work at the site.

French insurance firm AXA said on Tuesday that it provided insurance coverage for two of the contracting firms working on Notre-Dame's restoration prior to the blaze.

In a statement, AXA said its staff were cooperating fully with investigators.

The French insurer also said it provided insurance coverage for some of the relics and religious artworks displayed in the cathedral.

An AXA spokesman declined to estimate the company's potential liabilities associated with the damage caused.

The French state's policy is to bear the cost of reconstruction of historical monuments such as Notre-Dame Cathedral in the event of disasters.