Gacy was executed in 1994 for the murders of 33 young men. Investigators were only able to identify 25 of those victims.
Gacy took with him to the grave any information that might have helped authorities learn more about the other eight, but now, a lab in Fort Worth, Texas may finally provide answers.
Police in 1978 had the horrific task of pulling bodies from the crawl space and other areas around Gacy's home: bodies of young boys whom Gacy had lured with the promise of a job or by using a badge.
The so-called "clown killer" would eventually be convicted of killing 33 people, most of whom investigators identified using dental records, the most sophisticated method available at the time. That worked for all but eight victims who remain unidentified to this day.
Now, in the heart of cowboy country, Fort Worth, Texas, researchers are working to help Chicago-area families find long-awaited answers.
The University of North Texas Health Science Center is using some of the most sophisticated DNA identification technology in the world to try to learn the names of those final eight victims.
"[It's] not so much that it was high-profile serial killer, but the fact is these families have gone this long without knowing," said Dr. Arthur Eisenberg of University of North Texas Health Sciences.
This summer, the Cook County Sheriff's Office exhumed the bodies to extract DNA samples. The bodies are more than three decades old, and DNA decomposes along with the rest of the body. The samples are microscopic. There is no lab in Illinois capable of doing this work.
"Without the university coming forward like they did... I don't know how we would have done this," said Cook County Sheriff Tom Dart.
DNA from the bodies is just half the equation. The sheriff is collecting DNA samples from family members with loved ones who went missing during Gacy's six-year killing spree in the 1970s.
The lab is comparing the samples, hoping to find a match.
"Our job is to provide information. If that information can give these families peace, or at least allow them to wake up each morning now knowing what 's happened to their loved one, that's the first step," said Eisenberg.
Most of the university's funding comes from the federal government, like the Department of Justice, and the DNA samples go into a computer database. That means even if the samples provided by the families fail to match the eight Gacy victims, they could match one of the estimated 40,000 other unidentified bodies around the country.
"If their samples can get into the national databases, then their cases are active. They're not cold," said Eisenberg.
Since reopening the search for the Gacy victims' identities, investigators have found they are helping families in ways they never anticipated.
Harold Wayne Lovell's family long thought Gacy killed their brother, but as they prepared to send a DNA sample, they discovered he was still alive, living in Florida.
Other samples the lab determines are not a match for Gacy victims could still provide information.
"In some instances, the results are gonna be, we still haven't found your loved one, but he was not one of his victims, which should be some level of relief," said Dart.
The lab has finished its work on the first few samples from relatives. The sheriff and the medical examiner will determine whether they can finally give a name to any of the eight unidentified victims, which could happen within the next week or so.