The investigators told WBBM-TV, Chicago, Gacy's DNA profile has been entered into the FBI's 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," Cook County Sheriff Tom Dart said.
Dart's office found records at Gacy's home that indicated he traveled widely, raising the possibility of out-of-state victims.
"We've already placed him in ... 15 or 16 different states, definitely Canada," Dart said.
Dart and the FBI in Chicago are working to determine whether there are unsolved homicides in the areas where Gacy traveled.
Gacy was executed in 1994 for the killing of at least 33 young men and boys. Investigators have speculated Gacy may have been involved in 200 deaths.