United 5573 bomb threat forces flight to land in Phoenix All 46 passengers and flight crew evacuated from flight 5573 Bomb squad swept plane on a runway at Phoenix Sky International Airport Officials refuse to elaborate on nature of threat FBI now taken over investigation into incident A bomb scare forced a pilot of a passenger jet to perform an emergency landing after crew discovered a suspicious package on board. All 46 passengers and crew evacuated the plane moments after the flight from San Francisco to San Antonio touched down at Phoenix Sky International Airport. The bomb squad and a team of police dogs then searched Flight 5573 for dangers for several hours yesterday evening. Grounded: Flight 5573 sits on the runway of Phoenix Sky Harbor International Airport as bomb squad officers try to find out what threat is posed by a suspect package The United Express flight was held on the runway for four hours by Transportation Security Administration officials. Phoenix police spokesman Sgt. Steve Martos said: 'This is one of those types of incidents where, out of an abundance of caution, we would prefer to go through his tedious process to ensure there's no actionable, viable threat rather than let it continue and something actually occur. Any threat that we receive, we are going to take extreme caution and extremely seriously. Safety: Passengers are led a safe distance from the plane which was forced into an emergency landing after a member of the flight crew discovered a suspect package on board He said that a bomb squad and police dogs searched the plane and luggage 'to make absolutely sure there's no threat,' and nothing dangerous was found. Passengers were screened by bomb squad officers for any explosive substances and all were found to be clear. They were allowed to continue with their journey later yesterday evening. The mid-air drama unfolded yesterday afternoon when one of the flight attendants found what was thought could be a bomb. Hunt for clues: Bomb squad and TSA officials grounded the plane for four hours while they swept the jet for possible threats But officials are remaining tight-lipped about the specifics of the threat or nature of the package. Officials at Phoenix Sky Harbor International Airport said the plane landed at 4:45 p.m and had been given the all-clear by 7.15pm. Sky Harbor spokeswoman Deborah Ostreicher told the Associated Press: 'No further information is available about the specific security issue that caused the flight to be diverted to Phoenix, but the passengers are all safe and police are investigating.' The plane that was searched was isolated on the northwest side of airport and Sky Harbor officials said the incident wasn't causing any flight delays. United Express is the brand name for several regional airlines that operate shorter flights for United Airlines. The FBI has now taken over the investigation in to the incident.