A flight from California to Nevada was diverted after a passenger found a threatening note scrawled on two tray tables.
The message was discovered after the Southwest Airlines Boeing 737 took off from Orange County on a flight to Las Vegas. Officials would not elaborate on its contents.
The flight's 136 passengers were escorted to a secure area in the airport, while authorities inspected the jet.
Southwest spokeswoman Beth Harbin said nothing threatening was found.
Passengers were still waiting for the flight to resume nearly seven hours after landing in Ontario, about 65 kilometres east of Los Angeles.
Also on Wednesday, a flight operated by a regional carrier for Northwest Airlines was delayed in Oklahoma City after a woman told authorities there was a bomb on the plane.
No bomb was found, and the woman who made the statement was being evaluated, police said.
The flight to Memphis, Tennessee, was ready to depart when the woman made the threat to a flight attendant, said Karen Carney, a spokeswoman for Will Rogers World Airport.