One Sunday morning before church, when Welles Crowther was a young boy, his father gave him a red handkerchief for his back pocket. Welles kept it with him that day, and just about every day to come. When the Twin Towers fell, Welles’ parents had no idea what happened to their son, who had taken a Wall Street job there. Eight months after the attacks, his mother read a news account from several survivors who said they and others had been led to safety by a stranger, carrying a woman on his back, down nearly 20 flights of stairs. They didn’t know his name, but despite the smoke and panic, one of them remembered a single detail clearly: the man was wearing a red bandanna.