A commuter airplane flying from Philadelphia to Houston was diverted to a Tennessee airport Monday after a threatening note was found on board, authorities said. None of the 56 passengers aboard US Airways Flight 3441 was injured, and no bomb was immediately found, authorities said. The regional jet landed without incident at the Tri-Cities Regional Airport in eastern Tennessee about 11:30 a.m., according to airport spokeswoman Melissa Thomas. Transportation Security Administration spokeswoman Amy von Walter said a threatening note was found on board, and FBI, TSA, canine bomb-sniffing teams and local law enforcement were investigating. Details about the note's contents were not released.