The alert happened at Terminal C at New Jersey's Newark International when a man apparently passed into the so-called "sterile" area without being screened, Transportation Security Administration (TSA) spokeswoman Sari Koshetz said.
"A man was observed entering the sterile area of the airport from the exit lane, and we immediately froze operations," she said.
The alert came on the same day that TSA toughened security measures for all passengers on US-bound flights following the Christmas Day failed attack on a Northwest Airlines plane flying into Detroit.
Working with law enforcement agents, Newark officials pulled out the security tapes to see if they could identify the man who triggered the scare around 5:30pm (2230 GMT). It was not clear almost four hours later whether he had been tracked down or not, Koshetz said.
But all passengers waiting to board flights at the terminal, mainly served by Continental Airlines, were being re-screened as officials reviewed the security tapes.
"To be cautious, we wanted to make sure that everybody who gets on a plane tonight has been screened," Koshetz added.
Under the new TSA rules, all passengers arriving from abroad on US-bound flights could face random screening, while travelers from 14 "terror-linked" countries will also have to undergo mandatory enhanced screening.
The scare at Newark, one of three international air hubs in New York, was adding to flight delays already caused by high winds and freezing temperatures. But the airport's other terminals were not affected.
Newark's press office did not immediately reply to a request for information.
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