Authorities believe the substance that sickened two customs agents at JFK Airport this morning was ordinary nail polish remover, a law-enforcement source told The Post.

The incident, which forced the evacuation of two buildings near the airport’s perimeter, does not appear to be terror-related, the source said.

It was unclear why several tests at the scene produced consistent results indicating the presence of the highly lethal nerve agent VX, which kills through skin contact or inhalation and is used in weapons of mass destruction.

“The FBI responded to John F. Kennedy airport earlier today following reports of two sick employees. The FBI screened and tested the employees and the package they opened. The package in question was determined to be beauty supplies and nothing further. The scene has been cleared by FBI personnel,” said FBI spokesman J. Peter Donald in a statement.

Both victims suffered respiratory distress after they were overcome by fumes inside a US Postal Service facility at 250 North Boundary Road around 9:40 a.m.

A source said the substance may have been released when its packaging broke in transit.

The scene of the incident is a giant facility that processes overseas mail bound for the eastern United States, and a source said the package involved was believed to have come from China.

The building and a US Customs facility about a quarter-mile away were both evacuated and quarantined, but airport operations were unaffected.