The highly lethal nerve agent VX — used in weapons of mass destruction — appears to have been discovered in a mail facility at JFK Airport where two US customs agents were sickened Sunday morning a law-enforcement source said.

Both victims suffered respiratory distress and were taken for emergency treatment after being overcome while inside the US Postal Service facility on North Boundary Road at 9:40 a.m., the source said.

Initial testing after the incident revealed the presence of “chemical-grade weapons and nerve gas,” and subsequent testing produced a positive reading for VX, the source said.

VX is a synthetic substance that is the most potent of all known nerve agents and can cause death through skin contact or inhalation.