John Wayne Gacy was convicted in 1980 of killing 33 men and boys in the Chicago area between 1972 and 1978.

Authorities say Gacy, a contractor and part-time party clown, lured his victims with promises of construction jobs, drugs, alcohol or money for sex, or by posing as a police officer.

Most of the victims were strangled or asphyxiated, with many of their bodies hidden in the crawl space of his home and at least four others dumped in the Des Plaines River. Using the scientific methods available at the time, investigators were able to identify 25 of the victims, but the rest could not be identified.

Gacy was executed in 1994, but nearly 34 years after his arrest, one major question still remains: who were those eight unidentified victims?

In early 2011, Cook County Sheriff Tom Dart directed his officers to focus on solving some of their cold cases. Soon afterward, Detective Jason Moran came to him and informed him that the mystery of those eight Gacy victims was the largest cold case in county history.

Last October, Sheriff Dart announced that his office had exhumed the victims' remains in the hope of using modern DNA testing to do what detectives could not do decades earlier and provide closure for the victims’ families. Since then, the investigation has grown to include an extensive review of decades-old evidence and has raised even more questions about Gacy and his crimes.

“It’s sort of taken on a life of its own,” Dart told HLN one year after the announcement.

So far, the sheriff’s office has received more than 150 leads regarding people who could potentially have been Gacy victims. 71 have been cleared and closed, including four people who were found alive and two who turned out to have died from natural causes years ago.

34 leads are currently being investigated and require further action. In some cases, that means waiting for results from the laboratory at the University of North Texas Center for Human Identification, which is handling the DNA testing. In others, detectives are seeking more information from the missing people’s families.

36 leads have not been processed at all yet “because of manpower issues,” Dart said.

The sheriff’s office has had two major successes since the new investigation began last year. The first was confirming the identity of one of the eight Gacy victims.

William “Bill” George Bundy was reported missing in October 1976. According to a sheriff’s office press release, one of Bundy’s friends believed he may have worked construction jobs for Gacy, and his family had always suspected that Gacy killed him.

In 1979, following Gacy’s arrest, Bundy’s mother tried to obtain dental records for detectives to compare against the unidentified remains, but his dentist had retired and destroyed his records.

After Dart announced the renewed effort to identify the victims in 2011, Bundy’s sister contacted authorities and offered to provide a DNA sample. As a result, Bundy was identified as victim #19, the 19^th body removed from Gacy’s crawl space.

“I thought if we got one, that would be a success,” Dart said. After the identification of Bundy, though, “we’ve gotten somewhat greedy thinking we should be able to get more.”

A more unexpected victory came with the identification of Daniel Noe, a 21-year-old who vanished after telling his parents he was going to hitchhike from Bellingham, Washington to their home in Peoria, Illinois in September 1978, according a sheriff’s office press release.

Given the possibility that he would have passed through Chicago on that journey, a relative contacted detectives after learning of the new investigation. Since he fit the general profile of Gacy’s victims, the sheriff’s office obtained DNA from his residence.

While the results showed that he was not among the victims, a genetic association was found to a body discovered by hikers on Mount Olympus in Utah in 2010. Additional testing confirmed that it was Noe, who had always enjoyed the outdoors and mountain hiking. Foul play was not suspected in his death.

Dart said the odds of Noe’s case playing out as it did were “absolutely astronomical.” He was pleased that his office was able to give Noe’s family the peace of mind of knowing for certain that their son was not among Gacy’s victims.

He pointed to the case as a perfect example of the benefit of contacting his office and submitting DNA for families who believe there is even a chance their relative was a victim of Gacy.

“You just cannot tell what you’re going to end up finding out,” he said.

Surprisingly, the complex DNA testing has been the easy part of the investigation. The biggest challenge, according to Dart, is “how many doors this has opened up…How many bizarre directions this has taken us in.”

Detectives have been reviewing the original Gacy case files, “reams and reams of records” that are more than 30 years old, Dart said. This includes evidence that was not even presented at Gacy’s trial because it was not necessary for his prosecution.

Law students from Notre Dame are assisting with that effort. Dart said the process is time-consuming but it has “opened up these other areas where we had to investigate.”

According to Dart, investigators have uncovered “bizarre patterns” in the records that suggest Gacy could have had even more victims, including some outside the Chicago area. Evidence shows that Gacy traveled a lot more than had previously been thought, so detectives are now looking at people who went missing in the areas he visited.

Dart said common sense and evidence lead him to believe Gacy would have continued killing when he left Chicago.

“He turns off his serial killer mode when he leaves town?” he said. “That doesn’t make any sense.”

Dart suggested that a “monster” like Gacy who committed murders in Chicago so often could have been more emboldened when he was out of town, and he finds it hard to imagine that additional victims will not be found as detectives look into Gacy’s travels.

The investigation has also turned up additional evidence linking Gacy to an address on the north side of Chicago that Dart has wanted to search for a while. Dart said he is now in the process of trying for a third time to get a warrant to look for bodies buried there.

The property was previously searched with the owner’s consent in 1998. According to the Chicago Tribune, two spots were excavated at the time and no bodies were found, but radar scans showed more than a dozen anomalies under the ground.

“Am I making a leap that there’s bodies there?” Dart said. “No, I’m not.”

But he feels the possibility is worth exploring, and he said it could be done much less intrusively than in 1998 with new technology. Prosecutors have denied previous requests for a search warrant from the sheriff’s office, saying they lacked probable cause.

Still, identifying the other seven victims remains a priority. Dart wants to get as many people with missing relatives as possible to come forward and provide DNA.

The potential victims are described as white males, ages 14-23, who disappeared between 1970 and December 1978. Anyone with lost family members who fit that description is asked to contact the Cook County Sheriff’s Office at (708) 865-6244.

The sheriff’s office has also set up a webpage with the known details about each of the seven remaining victims, including their approximate heights and ages, and an online form that families can use to submit information.