On further inspection they discovered a container of other similar devices, along with what appeared to be detonators, said Frederic Petit, who represents the company's employees for the CGT union.
The staff alerted authorities of "imminent danger," and officials arrived to test the device using sniffer dogs and X-ray machines.
An airport employee at the airport said the devices were actually decoys bound for the US embassy in Tunisia that were being used for a training exercise.
"This type of delivery is not common, but sometimes takes place. This is just the first time that a package has been opened,” the employee said.
But Petit slammed the exercise as "irresponsible," given the heightened security concerns since the Paris attacks in November, when jihadists killed 130 people.
"Nobody was aware of this cargo," he said, adding that FedEx employees want to see such parcels banned from transit through France.