Authorities say the bodies were found in a remote wildlife area by hunters.
Black Hawk County sheriff's Captain Rick Abben said their confidence is based on evidence at the scene and the preliminary investigation. However, autopsies are still pending.
Lyric Cook and Elizabeth Collins vanished while riding bikes in Evansdale in July. Hunters found the bodies about 25 miles away from their home.
Local and state police and the FBI joined in a massive search that continued for months after the girls disappeared. No one has been charged in the case.
A number of investigators are gathering evidence from the area where the bodies were found.