Californian inmate Samuel Little has been serving time in a California prison since 2014, charged with the deaths of three women who were beaten and dumped in the late 1980s.
Now, the 79-year-old is claiming he is responsible for the deaths of 90 women.
A Texas prosecutor said on Friday that investigators have linked more than 60 killings in at least 14 states to Little, who may be the most prolific serial killer in U.S. history, reports news.com.au.
Ector County District Attorney Bobby Bland said Samuel Little continued to co-operate with investigators from around the country who were interrogating him in prison about cold case killings dating back to the 1970s.
Among those who spoke to him were investigators from Ohio, where Little grew up and where he's suspected of killing at least five women.
Little was convicted of killing three women from the Los Angeles area and also pleaded guilty to murderig a Texas woman. He's currently serving life sentences.
Little, who lived a nomadic lifestyle, claims to have killed 93 women as he crisscrossed the country over the years.
Bland said Little is now in failing health and has exhausted his appeals, leading him to be forthcoming with investigators.
"At this point in his life I think he's determined to make sure that his victims are found," he said.
Samuel Little was arrested at a Louisville homeless shelter in 2012. He was extradited to California on drug charges, and once there his DNA was quickly matched to three unsolved homicides in the 1980s.
During Little's 2014 trial in Los Angeles, prosecutors said he was likely responsible for at least 40 killings since 1980.
Authorities at the time were looking for possible links to deaths in Florida, Georgia, Kentucky, Louisiana, Mississippi, Missouri, Ohio and Texas.
But Little wasn't co-operating with investigators back then, and Bland credits Texas Ranger James Holland with gaining Little's trust and eventually eliciting a series of confessions.
Mr Holland travelled to California last year to speak with Little about cold cases in Texas.
That led Little to be extradited to Texas, and he plead guilty in December over the 1994 strangulation death of Denise Christie Brothers in the West Texas city of Odessa.
But Mr Holland's conversations with Little have continued, even after he was returned to California to serve his sentences. It was Mr Holland who determined that he was responsible for 93 deaths.
Information provided to Mr Holland was relayed to law enforcement agencies in several states, leading to a revolving door of investigators who travelled to California to corroborate decades-old deaths.
Among them were investigators from Ohio, where prosecutors on Friday announced charges against Little in the 1981 killing of a Cincinnati woman and where he was charged last week in the deaths of two more women in Cleveland.
He previously was charged in a second Cincinnati killing and confessed to another one in Cleveland, though investigators are still trying to identify the victim in that case.
Little's victims were often suffocated or strangled, in many cases leaving few physical marks and leading investigators to determine the women died of overdoses or of natural causes.
"There's still been no false information given," Mr Bland said.
"Nothing has been proven to be false."
Gary Ridgway, the so-called Green River Killer, pleaded guilty to killing 49 women and girls, making him the most prolific serial killer in US history in terms of confirmed victims, although he said he killed 71.
Little reportedly remembers the faces of the women he killed well enough to sketch them decades later, but the names elude him.
The FBI is hopeful the portraits of women he has drawn will help authorities to identify them and solve other cold cases.