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French police scientists and other expert investigators are starting to examine Notre Dame Cathedral for the first time since last week's devastating fire.

A police official said teams from three different police agencies are entering the monument Thursday to take samples and search for clues as to what caused the fire. Police made a preliminary visit last week but weren't able to conduct a thorough examination until the cathedral's structure was secured.

Officials believe the fire was possibly caused by a short circuit linked to renovation work underway before the fire. The Paris prosecutor's office says all potential leads are being pursued.

The cathedral will remain closed to the public for years after the fire destroyed its roof and knocked over its spire.