It would take only 118 pounds of fentanyl to kill 25 million people.

Illustration: Joseph P. Kelly

That’s how much of the powerful opioid painkiller Nebraska State Trooper Sam Mortensen found in April when he stopped a truck marked “U.S. Mail” swerving onto the shoulder along Interstate 80.

Rolling up the trailer door revealed an empty hold. But just below a refrigeration unit, behind a plastic panel secured with mismatched bolts, Mortensen found 42 brick-shaped packages, weighing 54 kilograms, full of fentanyl. The...