SALT LAKE CITY, Utah (AFP) - A United Airlines passenger jet was forced to make an emergency landing in Salt Lake City on Thursday after an undisclosed security alert, federal officials said. Federal Aviation Administration (FAA) spokesman Ian Gregor said United flight 741 from Denver to San Francisco landed safely and uneventfully at Salt Lake City International Airport at around 9:45am (1645 GMT). The FAA would not comment on the nature of the security alert although local media reported the scare was triggered after the discovery of a note containing a bomb threat. The 193 passengers and six crew were ushered off the plane as FBI agents, police and sniffer dogs began combing the aircraft. The incident comes amid heightened aviation security in the United States following the failed Detroit bombing in December.