Gacy is believed to have murdered at least 33 men between 1972 and 1978. Some 40 years later, six of those men are still unidentified.
But one family was brought closure Wednesday, as Cook County authorities identified James "Jimmie" Byron Haakenson as another victim of the so-called "killer clown."
Haakenson was 16 years old when he left his home in St. Paul, Minnesota, in search of a different life in a bigger city in 1976, Cook County Sheriff Thomas Dart told reporters Wednesday.
The teenager came to Chicago in early August and called his mom on August 5, 1976, to let her know he had arrived. It would be the last time she would hear from her son.
Police believe Gacy murdered Haakenson shortly after he made that phone call home, quite possibly that same day.
It is not known how Haakenson and Gacy met. Gacy was known for searching areas around Chicago looking for men who were gay, alone or looking for work.
Gacy would lure men to his home on false pretenses, often offering them rides, money, drugs, alcohol or a job. He would then impair them before sexually assaulting, torturing and killing them.
Authorities used DNA to identify victim
By the time police uncovered the crawlspace in Gacy's home in 1978, Haakenson's body was unidentifiable. For 39 years, he was given a new identity: Victim No. 24.
In the 1970s, police could only identify victims using dental records. Cook County officials removed the jawbones from the eight unidentified Gacy victims before burying them in county cemeteries, Dart said.
County officials found the bones did not provide enough information for four of the unidentified victims, and in 2011, authorities exhumed the bodies to gather more DNA.
Dart said authorities gathered enough DNA evidence on the victims and are ready to start bringing closure to their families. Dart said he hopes more families will come forward in the near future so more victims can be identified.
Nephew of victim searched for answers
It was earlier this year when a nephew of Haakenson's reached out to the county to find out more about his uncle.
Dart said the nephew came across information on the county's recent efforts to identify the victims. Shortly after, he persuaded his father and aunt, Haakenson's brother and sister, to take a DNA test.
The DNA submitted by the family members was an "immediate hit" on Victim No. 24, Dart said, which quickly led to identifying Haakenson.
It wasn't the first time family members came forward to link Haakenson's disappearance to John Wayne Gacy.
Haakenson's mother went to authorities in 1979 to see if her son was a victim, Dart said. But due to limited resources at the time and the mother's lack of dental records, nothing was recovered. The mother passed away in the early 2000s, Dart said.
Timeline of murder determined from positions of victims
Cook County authorities used other victims' positions in the crawlspace of Gacy's suburban Chicago home to narrow the timeframe of Haakenson's death.
Haakenson's body was found between the bodies of two other men: Rick Johnston and another unidentified boy, referred to by police as Victim No. 26.
Johnston, whose body was found on top of Haakenson's, is believed to have been murdered by Gacy on August 6, 1976, after attending a concert at the Aragon Ballroom in Chicago.
Dart said authorities believe the third victim, whose body was found below Haakenson's, was murdered in July or August of 1976.
Police urge people to come forward
When remains were first uncovered in Gacy's home in 1978, eight victims were unidentified. But due to advancements in technology, that number has been reduced to six.
Dart said the victims were identified because "people agreed to come forward with DNA."
Authorities are now urging people who had loved ones who went missing in Chicago during that period to come forward and submit their DNA -- and hopefully get some answers.
"Every family deserves closure, without hesitation," Dart said.