Passengers who had already boarded planes at the Delta Air Lines terminal were removed, and flights out of the terminal were grounded.
Steve Coleman, a spokesman for the Port Authority of New York and New Jersey, which operates the region's airports, said passengers were required to pass back through metal detectors before boarding planes again.
New Federal Aviation Administration rules require such precautions in the wake of the Sept. 11 terrorist attacks.
Coleman said he was uncertain how many passengers were affected after the knife was found around 6:30 p.m. Monday.
The terminal reopened at 10:30 p.m., and service resumed shortly after, according to Port Authority spokesman Sgt. Jeff Baumbach.
Monday's incident was the latest in a string of evacuations at Kennedy following apparent security breeches.
On Nov. 7, a terminal was evacuated for 90 minutes after FAA officials found security at a passenger screening checkpoint was operating improperly. A week earlier, FAA security agents ordered the temporary evacuation of part of a terminal when they saw checkpoint screeners failing to properly search people who set off alarms.
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