CIUDAD JUÁREZ, Mexico—Two Americans were driving back to El Paso, Texas, last December after an afternoon across the border in Ciudad Juárez. A few blocks from the border, they were surrounded by Mexican army trucks and pulled from their Dodge Ram.

Mexico's military says it found two suitcases full of marijuana in the cab of the pickup truck. Two soldiers later testified that they drove the two Americans to a military compound on the outskirts of town, questioned them briefly, then turned them over...