The fake grenade -- which Port Authority Police flagged as being a possible explosive device -- was spotted passing through an X-ray machine in Tower 1's north lobby just after 9 a.m.
Responding officers quickly determined it was not a threat by a bomb squad.
However, the 24-year-old was taken into custody and Port Authority Police said charges are pending.
Investigators said there was no lockdown at 1 World Trade Center and the device was quickly cleared.
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