MAYWOOD, Ill. — It was a painful mystery that had simmered just below the surface for about 40 years, and last month, the family of James Byron Haakenson finally got their answer. As had been long feared, the funny, good-natured 16-year-old they called Jimmy had been a victim of John Wayne Gacy, one of the country’s most notorious serial killers.

But the revelation by Chicago-area law enforcement officials opened up a new set of haunting questions for this family as it imagined his final days, now with just enough certainty to be horrifying.

“How did this 16-year-old kid get to Chicago, and how in the heck did he run into this awful man?” Lorie Sisterman, Jimmy’s older sister, said from her home in North St. Paul.

The story of Jimmy’s identification, decades after his death, is a remarkable quest that spanned the country. It took a curious nephew in Texas with a knack for digging around online, siblings in Minnesota and South Dakota who had never stopped wondering what had happened to their brother, and a sheriff-detective team in Illinois determined to close cold cases.