Airport spokeswoman Victoria Lupica told The Associated Press that the meticulous examination of Flight 968 by law enforcement and explosives experts found nothing amiss and the plane would continue its journey to Alaska.
The Boeing 757 had landed in Philadelphia in the afternoon, from Glasgow, Scotland.
The 157 passengers and six crew members had boarded the Anchorage, Alaska-bound flight when they were asked by Transportation Security Administration officials to deplane so it could be swept by law enforcement officials, Tempe, Ariz.-based US Airways Inc. spokesman Andrew Christie told the AP.
The TSA said in a statement that the investigation was launched because of a threatening note, but it didn't provide details about it. Lupica said she didn't know what the threat was, but she said the incident was referred to the FBI.
The passengers were taken inside Terminal A, one of the airport's seven terminals. Across the tarmac, baggage handlers unloaded the Boeing 757.
Outside the plane, an explosive ordnance removal technician X-rayed at least one suitcase. Later, a police officer took the suitcase and dumped its contents onto the ground.
Other items aboard the plane, which also was carrying commercial cargo, were examined, too.
Passengers inside the terminal who were continuing on to Alaska were interviewed by investigators.
No delays were caused by the plane evacuation, and no injuries were reported.
Philadelphia International Airport, which handled 31 million passengers last year, is the only major airport serving the Philadelphia area, one of the largest metropolitan areas in the United States. Airlines that use Terminal A include Lufthansa, British Airways, US Airways, Air Jamaica and Frontier.
In June, flights were grounded at Reagan National Airport, near Washington, D.C., after a woman with a history of mental health problems told a US Airways ticket agent in Dayton, Ohio, that there was a bomb aboard a plane. The woman later said the warning came to her in a message from God, the director of Dayton International Airport said.
No explosives were found aboard the jet that flew from Dayton International to Reagan, and the woman was taken to a mental health facility.