Field tests showed an initial finding of nerve gas, but the FBI tested the substance in "an abundance of caution".
Despite the alarming first test, officials cautioned that false positives do happen. By late afternoon, experts confirmed that the substance was definitely not nerve gas.
ABC News has learned that the substance was some kind of nail polish remover. The nail polish remover was apparently being shipped from Long Island to Australia when it was opened and inspected at the airport. Some of the bottles had cracked in shipping, and when the workers opened the box, the fumes overwhelmed them.
It is not clear why the remover made the workers feel sick, and the passengers were not affected in any way. The incident happened in a remote part of the airport, and did not make a dent in the flight schedule.
The Port Authority of New York and New Jersey, which oversees the airport, didn't respond to a call seeking comment.
Customs and Border Patrol referred questions to the FBI.
The condition of the customs agents is not yet known.
The processing facility is the central mail facility for all overseas mail coming to the East Coast.