SAN ANTONIO (AP) — The tractor-trailer was pitch-black inside, crammed with maybe 90 immigrants or more, and already hot when it left the Texas border town of Laredo for the 150-mile trip north to San Antonio.

It wasn't long before the passengers, sweating profusely in the rising oven-like heat, started crying and pleading for water. Children whimpered. People took turns breathing through a single hole in the wall. They pounded on the sides of the truck and yelled to try to get the driver's attention. Then they began passing out.

By the time police showed up at a Walmart in San Antonio around...