It’s a nightmare “what if” scenario: Police intercept terrorists transporting nerve gas in a moving van just off the Long Island Expressway.

After yesterday’s weapons of mass destruction role-playing exercise in Old Bethpage, Mark White, a deputy chief with the Suffolk County police, said authorities learned “how we’ll respond if something gets through.”

The National Center for Biomedical Research and Training from Louisiana State University lead the morning exercise involving some 250 law enforcement officers, firefighters and medical responders.

Don Kelly, an NCBRT adjunct instructor, said it was a realistic training scenario in “a target-rich environment.”