Officials at Phoenix Sky Harbor International Airport said Flight 5573 landed at 4:45 p.m. and everyone on board was evacuated. 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 flight.
Phoenix police spokesman Sgt. Steve Martos said "the reported threat was on the plane" but he couldn't provide any details.
An unnamed government official said someone found the word "bomb" in the lavatory of the plane, according to ABC News. The official said passengers were screened and the plane was checked, but nothing was found.
United was making arrangements to get passengers on another flight to San Antonio. Sky Harbor officials said the incident did not cause any flight delays.
United Express is the brand name for several regional airlines that operate shorter flights for United Airlines.
ABC News and the Associated Press contributed to this report.