John Feal, now 54 years old, was a supervisor at a demolition company when terrorists hijacked two planes that brought down the World Trade Center buildings—and two others that crashed into the Pentagon and a field near Shanksville, Pa., respectively—20 years ago. Feal, who hails from Long Island, N.Y., arrived the day after New York City’s iconic buildings came down. He had been working at Ground Zero for five straight days...