BRUSSELS — For 20 years, the Dutch police investigated the rape and murder of an 11-year-old schoolboy, and the Dutch news media chronicled the case’s many twists and turns . Now, after an investigation in which 17,500 Dutchmen voluntarily took part in DNA profiling, a suspect has been arrested in Spain. The boy, Nicky Verstappen, had attended a summer camp at a nature reserve in the southern Netherlands in 1998. On a night that August, he left his tent and never returned. His body, pajama-clad and barefoot, was discovered the next day, hidden in a dense pine forest about a...