The Eiffel Tower was shut to all visitors on Sunday after three "terrorist suspects" with "large rucksacks" were seen ascending France’s most popular tourist attraction.
Anti-terrorist police supported by a helicopter could be seen at the iconic landmark following the alarm being raised in the early hours.
But after a search which went on all morning it was thought they escaped via parachute -- 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 5.30 a.m.," 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.
"There were, of course, fears that they may have left dangerous material on the tower before leaving."
According to 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 nearby River Seine.
There have been numerous bomb alerts at the Eiffel Tower in recent years, and France is currently on the highest state vigilance alert.
The 1,050 ft-tall iron lattice tower was built for the 1889 World’s Fair, and soon turned into a prestige symbol of modern France.
It is the most visited paid-for monument in the world, with some seven-million people a year going up it.