"On March 29, 1975, sisters Katherine and Sheila Lyons, age 10 and 12, vanished from a shopping mall in suburban Washington, D.C. As shock spread, then grief, a massive police effort found nothing. The investigation was shelved, and mystery endured. Then, in 2013, a cold case squad detective found something he and a generation of detectives had missed. It pointed them toward a man named Lloyd Welch, then serving time for child molestation in Delaware....