BROWNSVILLE, Texas—Some 1,000 trucks a day arrive from Mexico at a border checkpoint here near the southern tip of Texas. Their first stop is a white-and-yellow painted metal portal where energy beamed at the trucks produces X-ray images to try to spot fentanyl and other contraband. Across the southern border, most U.S.-bound trucks and nearly all passenger vehicles are generally scanned selectively if they are pulled aside. Mexican cartels have long profited from these odds while smuggling fentanyl and other narcotics. Their illicit trade propelled the U.S. to a record of nearly 107,000 fatal overdoses in 2021, the most recent year of full data.

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