CHICAGO — Just a few grains of the synthetic fentanyl being found in packages coming into the United States through this mail sorting facility are enough to slow a person’s heart rate, inhibit breathing, and send him or her into a coma. It’s the most dangerous kind of fentanyl in America.

Francis Byrne, an officer with U.S. Customs and Border Protection, is wearing latex gloves and pulls a couple of two-inch-by-two-inch baggies out of a white envelope that is no more than half an inch wide. He holds the bags at eye level. The see-through zipped-up bags are both full of white powder. He lowers it and hooks it up to a handheld digital machine that will tell him what’s inside.