The Eiffel Tower in Paris was shut to visitors on Sunday after three "suspected terrorists" were spotted climbing the structure with "large rucksacks."
Anti-terrorist police supported by a helicopter could be seen at the iconic landmark after an alarm was raised in the early hours.
But after a search which went on all morning it was thought they escaped using parachutes -- prompting a theory that they had been extreme sportsmen all along.
"There were reports of three people climbing the tower from the outside from about 5.30am," said a police source.
"They were said to have large rucksacks so no chances could be taken. They completely disappeared, so enquiries are centred on them being extreme parachutists," the source added.
Police also tried to find out whether the three left any dangerous material on the tower before leaving.
According to the Daily Mail online, the tower has frequently been threatened by terrorist groups, including al Qaeda and IS, with security stepped up since attacks by three radical Islamist gunmen in the city in January.
A police cordon was formed around the tower and people were moved to the banks of the River Seine nearby.
There have been numerous bomb alerts at the Eiffel Tower in recent years, and France is currently on the highest state vigilance alert.
The 324 metre iron lattice tower on the Champ de Mars was built for the Universal Exposition of 1889, and soon turned into France's most iconic landmark.
It is the most visited paid-for monument in the world, with nearly seven million people a year going up it.