Authorities on Sunday evacuated Mont Saint-Michel, one of France's busiest tourist sites, and were searching for a suspect who allegedly threatened to attack local security forces.
A police source said a tourist guide had reported the man, who was travelling on a shuttle bus carrying visitors to the rocky islet, after he made threatening comments about security forces.
Police tracked the man's first movements on CCTV after the tourist guide and several other people raised the alarm at about 07:45 (05:45 GMT), but they lost sight of him, triggering the evacuation.
Police described the operation as a "precaution".
As a police helicopter swirled overhead hunting the suspect, tourists were blocked from entering and holidaymakers were evacuated from hotels in an operation that went off calmly.
Mont Saint-Michel is about 1km off the north-western coast of Normandy and is attached to the mainland by a bridge.
The Unesco World Heritage site, which is topped by a medieval monastery, draws more than a million visitors a year.
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