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French Culture Minister Franck Riester says that a month after a fire engulfed Notre Dame Cathedral, the edifice is still being made safe enough for its restoration to begin.

Riester and Canadian Prime Minister Justin Trudeau wore hard hats for a visit Wednesday inside the Gothic church.

Trudeau said he wanted to show Canadians' solidarity "toward our French cousins." After the devastating April 15 fire, he said, one "can't help but marvel at how so much was saved."

The French culture minister said that operations to secure and conserve the cathedral must be completed before restoration works can start.

He said robots, for instance, were removing gravel from inside the cathedral, to be examined both by ministry experts and police seeking clues about the fire's origin.