Migration to US is risky. Why do parents let their teens try it solo?

A 13-year-old Guatemalan was on his way to reunite with his father in the United States. A pair of 16-year-old cousins from Mexico were looking forward to steady work. Two young Hondurans promised their mother the journey would set them up to build her a home one day.

Instead, these youths – along with dozens of adults – perished in an abandoned, sweltering tractor-trailer in San Antonio last week, dubbed the deadliest human-smuggling incident in modern U.S. history.

The tragedy underscores the growing risks migrants face in trying to reach...