PHOENIX >> No arrests have been made but an investigation is ongoing into a threat that came from on board a United Express flight that was then diverted to Phoenix and evacuated, authorities said today.

FBI spokesman Manuel Johnson said a crew member on Flight 5573 from San Francisco to San Antonio discovered the threat Thursday. He declined to provide any details on it, citing the continuing investigation.

The regional jet carrying more than 40 people landed at 4:45 p.m. at Phoenix Sky Harbor International Airport, and everyone on board was evacuated, airport officials said.

A SkyWest spokeswoman referred all questions about the threat to city authorities. Phoenix police spokesman Sgt. Steve Martos said “the reported threat was on the plane,” but he didn’t elaborate.

Martos said a bomb squad and police dogs searched the plane and luggage “to make absolutely sure there’s no threat,” and nothing dangerous was found. Martos said all the passengers were interviewed and cleared to continue their journey.

Officials with SkyWest Airlines Inc., which operates flights under the United Express brand, said there were 42 passengers, two pilots and two flight attendants on the jet.

The passengers departed on the same plane Thursday evening after they were rescreened by the Transportation Security Administration, officials said.

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.