The 78-year-old was spending the rest of his life in prison after being convicted of killing three people. But his name had popped up in the FBI's Violent Criminal Apprehension Program, known as ViCAP, in connection with a series of unsolved murders across the country.

One killing in Odessa, Texas, appeared to be particularly relevant, so two FBI crime analysts and James Holland of the Texas Rangers went out to see Little to try to get him to talk.

He was more than willing, according to the FBI

"Over the course of that interview in May," ViCAP crime analyst Christina Palazzolo said in an FBI article, "he went through city and state and gave Ranger Holland the number of people he killed in each place. Jackson, Mississippi -- one; Cincinnati, Ohio -- one; Phoenix, Arizona -- three; Las Vegas, Nevada -- one."

In all, Little confessed to about 90 murders in that interview and in others, according to the Texas Rangers and the FBI . The killings occurred across the country, from Los Angeles to Miami, Houston to Cleveland, all between 1970 and 2005.

Investigators have confirmed 34 of the confessed killings, authorities said. Many more are pending confirmation and a number remain uncorroborated.

This FBI map shows the locations where Samuel Little killed young women, according to his confessions.

Still, the stunning confessions could make Little the most prolific serial killer in US history if convicted, according to Ector County District Attorney Bobby Bland.

"There's been a lot of cooperation for law enforcement across the country as Texas Rangers are in the process of verifying the murders," Bland told CNN.

Little targeted marginalized and vulnerable women who were often involved in prostitution or addicted to drugs, the FBI article said. Palazzolo and Angela Williamson, a Justice Department senior policy advisory and ViCAP liaison, who both spoke to Little, said that he remembered great detail from the killings.

He remembered where he was, what car he was driving and could even draw pictures of the women he killed, the FBI said. He was less reliable on the dates of those incidents, however, the FBI said.

CNN has reached out to E.J. Leach, the attorney representing Little in the Texas case, and has not yet received a response.

How he got here

Samuel Little has confessed to 90 murders, according to authorities.

Little is serving three consecutive life sentences for murders he committed in Los Angeles County, according to Bland.

In the story on its website, the FBI said Little has previously faced shoplifting, fraud, drug, solicitation and breaking and entering charges in the past.

In September 2012, he was arrested at a Kentucky homeless shelter and extradited to California, where he was wanted on a narcotics charge, the FBI said. Once he was in custody, Los Angeles Police Department detectives obtained a DNA match to Little on the victims in three unsolved homicides from 1987 and 1989 and charged him with three counts of murder, the FBI said.

The victims, all women, had been beaten and then strangled, and their bodies were dumped in an alley, a dumpster and a garage, the FBI said. He pleaded his innocence but was convicted in 2014, the FBI said.

Authorities in Los Angeles passed his DNA on to the FBI's ViCAP program to get a full background on him, and ViCAP then reached out to the Texas Rangers to look into the killing of Denise Christie Brothers in Odessa.

Bland said Little provided details indicating that he was responsible for the death of Brothers, who was found strangled to death in a vacant parking lot two blocks away from a motel in January 1994.

A grand jury returned an indictment against Little, according to Bland, on July 16. Little was extradited from California to Texas to face charges in the Brothers case. He pleaded guilty to killing Brothers and was sentenced to life in prison, the Ector County District Attorney's Office said. The sentence will run concurrently with his sentences in California.

Over the course of the next several months, Little, also known as Samuel McDowell, cooperated with authorities, and provided additional details on approximately 90 slayings that Little said he committed, according to Bland.

James Holland, a Texas Ranger trained in cold cases, visited Little in a California jail, and over time, gained his trust, Bland said.

The Texas Rangers are still in the process of verifying many of the killings and none of the information that Little has provided has been disproved, Bland said. No formal charges on any of the additional killings have yet been brought.

Now in poor health, Little remains in custody in Odessa, and Holland has been conducting nearly daily interviews to get the most accurate and full picture of his confessions, the FBI said.

The Texas Department of Public Safety has declined comment.