A federal official confirmed to CBS News that the threatening note was in fact a "bomb threat." The official says that a search and K-9 check are being conducted "out of an abundance of caution."
None of the 56 passengers aboard US Airways Flight 3441 was injured, and no bomb was immediately found, authorities said.
The plane landed at the Tri-Cities Regional Airport in eastern Tennessee without incident 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.
Von Walter said passengers were taken to the airport terminal and were being interviewed by the FBI.
The Embraer plane was operated by Republic Airways, which is contracted by US Airways to run express flights for the company.
It was the latest in a series of recent flight diversions and delays in the United States and Ireland that included seven security incidents in one day. There was no indication terrorism was involved.
In one of those incidents on Friday, a stick of dynamite was found in a college student's checked luggage in Houston after a flight from Argentina.
The student was charged with carrying an explosive aboard an aircraft. His father said he believed the young man bought the dynamite as a souvenir when he visited a silver mine while traveling in South America.