Gacy raped and murdered young boys and men between 1972 and 1978 in Cook County. Investigators said he found some of his victims hitchiking and at bus stops.
The serial killer was arrested in 1978 and found guilty of murdering at least 33 young boys and men. Six of those victims are still unidentified.
"Their family members, their friends, they're still out there," said Carol Schweitzer, of the National Center for Missing and Exploited Children. "They're still alive, and somebody, I'm very confident, is going to be able to help recognize these two boys."
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The National Center for Missing and Exploited Children joined with the Cook County Sheriff's Office to release the two new facial reconstructions after remains were found under Gacy's home in a crawl space.
"Skeletal remains were given to us in a photographic form," said Colin McNally, also from the National Center for Missing and Exploited Children. "We're able to use PhotoShop and create what we call a photo composite, superimposing the facial features based on what the skull is telling us."
John Doe No. 10 was a white male between 17 and 21 years old. He was between 5 ft. 7 in. and 5 ft. 11 in. tall and his eye color and hair color are unknown. Investigators said he injured his left clavicle before he died. It had healed well over time. He also had dental fillings.
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John Doe No. 13 was a white man between 18 to 22 years old. He was between 5 ft. 9 in. and 6 ft. 2 in. tall and had dark brown, wavy hair. Investigators said one of his upper teeth was displaced behind another tooth. This may have been a distinguishing characteristic to people who knew him well.
"We just hope that these images will provide those answers to the people that are still looking and still searching," Schweitzer said.
Anyone with information regarding the unidentified boys should call the National Center for Missing and Exploited Children at 1-800-THE-LOST or 1-800-843-5678 or the Cook County Sheriff's Office at 708-865-6244.
The NCMEC also released video Monday, which highlights the work Cook County Detective Sgt. Jason Moran and Cook County Sheriff Tom Dart have done to help identify Gacy's victims.
WATCH: Gacy Unsolved
National Center for Missing and Exploited Children: Gacy Unsolved