Port Authority officials said the agents opened the package at a postal processing facility at Building 250, which is the central mail facility for all overseas mail coming to the East Coast. It was later determined that the package only contained beauty supplies.
Field air samples were taken, and preliminary results tested positive for "nerve gas." Additional tests were underway and FBI was called to the scene.
A law enforcement official told ABC News that several bottles of nail polish remover cracked inside the shipping container, and the agents were overwhelmed by the fumes. A source also said the package that originated from the U.S. and was on its way out of the country. The destination was not disclosed.
Despite the preliminary test results, authorities believed it was unlikely that actual "nerve gas" was present at the scene. They said it was likely a standard-use chemical that shouldn't have been in the mail - like a solvent or degreaser.
ABC News contributed to this report.