McCarran International Airport spokeswoman Candice Seeley says U.S. Airways flight 399 departed about 4:30 p.m. Saturday for Phoenix, but the pilot returned shortly after takeoff.
She says "the passenger made some sort of threat" after the plane landed, but she didn't know specifics.
Police were called and they met the aircraft.
Transportation Safety Administration spokeswoman Jonella Culmer says the passenger made an "inappropriate comment" as the plane taxied back to the gate.
Police and the FBI tell The Las Vegas Sun that they're investigating but no arrests have been made.