An apparent bomb threat made in connection with a trans-Atlantic flight forced law enforcement officials and emergency personnel to meet the jet on the tarmac.
USAirways Flight 777 from Shannon, Ireland, landed in an isolated part of Philadelphia International Airport shortly before 2 p.m. (1800 GMT) Wednesday. Passengers exited the Boeing 757 via staircases and were taken to waiting buses.
The airplane was searched and passengers and their luggage were being screened, a Philadelphia police department spokeswoman said.
Police and airport officials did not disclose any specifics about the apparent threat or details about from where it may have originated.
FBI spokeswoman Carrie Adamowski said agents were assisting Transportation Security Administration officials in the investigation.
U.S. customs officials have screening facilities at Shannon Airport and the passengers aboard the plane would have gone through them before boarding.
The plane, which had 171 passengers and nine crew members aboard, was supposed to continue to Pittsburgh after its scheduled stop in Philadelphia.
A spokesman for USAirways did not immediately return a message seeking comment.
FBI meets plane in U.S. after apparent threat