Updated 12/03/12 – 4:37 p.m.

CHICAGO (CBS) — Detectives have begun taking a new tack in their effort to pin cold cases on serial killers like John Wayne Gacy.

They used samples of Gacy’s blood to create a DNA profile. They can do this because Gacy is technically a homicide victim – since he was put to death in Illinois by lethal injection.

This latest effort started when Cook County Sheriff Tom Dart began exhuming the remains of unidentified Gacy victims to try to match their DNA to missing persons cases.

That got detective Jason Moran wondering: Why not do it the other way around?

So investigators have created a DNA profile of Gacy and other executed murderers, hoping to enter them into a national database that might link the killers to unknown victims.

“You just know some of these guys did other murders that were never solved,” Moran said.

“When you have someone as evil as this guy, a monster of this magnitude, to think he wasn’t doing things elsewhere really is quite naive,” Dart said.

Vials of Gacy’s blood, found in a sheriff’s department evidence box, were drawn from Gacy in March of 1979, three months after his arrest; but the Will County Coroner had a better-preserved sample, taken from Gacy before his 1994 execution.

That blood was processed by the Illinois State Police crime lab, and a DNA profile was developed and entered in the FBI’s combined DNA index system.

“Any DNA he left at any crime scene, wherever it is in the country, now would make a hit, and a case that may have occurred in the 70s could conceivably be closed out now,” Dart said.

Dart’s office also discovered something else: extensive records taken from Gacy’s home, showing he traveled widely, and suggesting the possibility of out-of-state victims.

There has been speculation that Gacy might have killed others whose bodies weren’t buried in the crawlspace under his home, or tossed into the Des Plaines River, as was the case with his known victims.

“We’ve already placed in him probably … 15 or 16 different states, definitely Canada,” Dart said.

Dart and the FBI have been working to determine whether there are unsolved homicides in those areas where Gacy traveled, at the times he was there.

Meantime, Dart said he’s already heard from one out-of-state sheriff who wants to perform DNA testing on evidence from two unsolved murders from the 70s, to see if Gacy might have been involved.

Gacy killed at least 33 young men. He was executed in 1994.

Watch & Listen LIVE