(CNN) -- A United Airlines flight originating in Denver, Colorado, was diverted to Salt Lake International Airport in Utah on Thursday after a threatening letter was found on board, airport public relations director Barbara Gann told CNN.
Passengers were being held in the airport's international terminal while the plane is inspected, Gann said.
The incident was not affecting other flights.
United Airlines Flight 741, a Boeing 757, was en route to San Francisco, California, CNN affiliate KSL reported.
The FBI, ATF, Salt Lake City Fire Department and bomb-sniffing dogs responded to the scene, KSL reported.