Officials at Phoenix Sky Harbor International Airport say Flight 5573 landed at 4:45 p.m. Thursday.
SkyWest Airlines, which operates flights under the United Express brand, says the 42 passengers, two pilots and two flight attendants aboard the regional jet were evacuated.
The nature of the threat wasn't immediately clear, but airport officials say a Phoenix Police Department bomb squad was called out to search the plane.
They say authorities were searching through luggage after passengers were bused to a waiting area in a terminal.
Airport officials said the passengers were being rescreened to resume their trip to San Antonio on Thursday night.
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