New York's LaGuardia airport was evacuated and flights were halted for about two hours after a man's shoe set off a false alarm during screening.
A search was launched for the man after he was allowed to pass through security despite setting off the alarm for explosives residue.
Officials could not say immediately why the man was allowed to pass through.
Hundreds of people were evacuated from the Delta Airlines terminal and departures temporarily suspended.
But incoming flights were allowed to land, said Yolande Clark, a spokeswoman for the federal Transport Security Administration (TSA).
Ms Clark said the man, who had been selected for secondary screening, afterwards "put on his shoes and exited the area".
Several hours after the alert was raised, the man had still not been found.
Andrea McCauley, another TSA spokeswoman, added that the screening alarm was sometimes triggered by substances other than explosives, including traces of fertilizer that can be found on the bottom of shoes.
Shoe checks have been routine at US and other airports since British "shoe bomber" Richard Reid tried to blow up a Paris-Miami flight in 2001 using explosives hidden in his footwear.
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