More than 100 passengers and five crew members were taken off US Airways Flight 1070 for more than four hours 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 J.J. 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. 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 and found none, Klaver told The Associated Press. The situation did not appear to have any connection to terrorism, he said.
The flight had been scheduled to depart at about 11 a.m. 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. The flight finally left after 4 p.m.
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.
Amanda Benner, who was on her way to Bermuda with her husband to celebrate their 15th anniversary, said the passengers were told only that there had been a "security breach" and were asked to leave everything on the plane and return to the terminal.
TSA personnel, police and "all kinds of guys in suits" quickly swarmed the plane, Benner said.
"They're very stern and serious," she said.
Associated Press writers Randy Pennell and Maryclaire Dale in Philadelphia and Eileen Sullivan in Washington contributed to this report.