Two US Airways workers loading a Bermuda-bound plane grew suspicious of whether another baggage handler was allowed be near the jet and confronted him, leading the man to flee and forcing security officials to remove passengers and rescan all their luggage.
More than 100 passengers and five crew members were taken off US Airways Flight 1070 for more than four hours on Thursday over the security issue.
The Philadelphia baggage handlers told their bosses when they saw a co-worker who was not assigned to be near the plane, FBI spokesman JJ Klaver said.
"When they asked him to leave, he drove away from the plane in a baggage loading-vehicle," he said.
The person had not been identified or located since, Klaver said, and officials believe he was authorized to be in that section of the airport, but not at that particular place.
Authorities swept the plane for explosives but found none, and the situation did not appear to have any connection to terrorism, he said.
The flight had been scheduled to depart at about 11am but passengers were instead removed and the plane was towed away for inspection.
The Airbus 319 was surrounded by emergency vehicles with lights flashing, and security dogs moved from bag to bag, checking luggage spread on the ground near the jet before it was finally allowed to leave after 4pm.
US Airways spokesman Todd Lehmacher confirmed the search but would not comment on specifics.
Transportation Security Administration regulations require airports to submit security plans that include access and perimeter control, and airports are responsible for day-to-day enforcement.