Investigators have been able to close four unsolved murder cases because of the confessions of Samuel Little.
Officers said Little provided details on two killings in Houma, Louisiana, which only the murderer would have known.
Dorothy Richard, 59, was found dead in 1982, and the body of Daisy McGuire, 40, was discovered in 1996. Both had been strangled.
Officers are also investigating his confessions to other killings.
In Macon, Georgia, the Bibb County Sheriff’s Office said in a statement issued on Friday that Little’s confessions allowed its investigators to close two murder cases, one from 1977 and another from 1982.
In the 1982 case, the body of Fredonia Smith was found in the backyard of a Macon home. She had been strangled.
The 1977 victim has never been identified. Her skeletal remains were found at the edge of some woods in a local backyard.
Two Bibb County investigators travelled to Decatur, Texas, to question Little.
The sheriff’s office said he gave them specific details and information linking him to both murders.
Smith’s family has been notified about the new developments.
Ector County district attorney Bobby Bland said one of Little’s confessions was about the February 1994 killing of Denise Christie Brothers in Odessa, Texas.
“Due to the dedicated and tireless work of Ranger Holland and the Texas Rangers, Little will be confirmed as one of, if not the most, prolific serial killers in US history,” Mr Bland said in a statement.
“As a result of this investigation, not only did the family of Denise Brothers get answers, but so will hundreds of family members know the truth after decades of questions.
“Justice for the victims of Samuel Little may have been delayed, but because of these efforts, justice is no longer denied.”