AFP 

WASHINGTON, Aug 29: A plane with 56 people on board travelling from Philadelphia to Houston was diverted on Monday after the discovery of a "threatening note on board," a spokesman for the Transportation Security Administration said.                

The plane, US Airways flight 3441, landed without incident at Bristol Tri-Cities Regional Airport in the southeastern state of Tennessee, he said.           

All 56 people on board were evacuated and explosives teams began to search the aircraft, the spokesman said.     

FBI agents were sent to begin interviews with crew and passengers, an FBI spokesman said.         

"We have agents out of our Knoxville office that have responded to the scene. We're interviewing passengers and investigating what has taken place," an FBI spokesman said.          

"The pilot reported suspicious activity on the plane, and the plane was diverted to the airport. As is usual, we are interviewing the passengers and crew members to determine what, if any, situation took place," the FBI spokesman added.                

The United States is under heightened security alert since British authorities announced an alleged plot to blow up US-bound airliners, and with the approach of the fifth anniversary of the September 11 attacks, and several scares have taken place.                

In a single day, Friday, there were seven such incidents, according to the FBI.