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A serial killer who has confessed to 90 murders has drawn 16 of his alleged victims as the FBI works to connect him to decades-old cold cases around the country.

Samuel Little Wise County Sheriff's Office

Samuel Little was convicted in 2014 of killing three women in California in 1987 and 1989. He was sentenced to three consecutive life sentences.

He had always maintained his innocence in those three cases, but last year, in exchange for a prison transfer, "Little was willing to talk," according to the FBI.

Little, 78, said he killed 90 women, and the FBI has been trying to line up those admissions with unsolved deaths in more than a dozen states from 1970 to 2005.

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Recently, Little drew sketches of 15 women he claims he killed. One of the victims he sketched has been identified as a man killed in Miami, called "Mary Ann" or "Marianne," according to the FBI.

At left is Little's sketch of a black woman in her late twenties who was killed in 1984. The FBI has matched the drawing with a Jane Doe, an unidentified victim. At right is Little's sketch of an 18-year-old black male, called "Mary Ann" or "Marianne." FBI

Shortly after Little's confessions in November, the FBI was quickly able to confirm he had killed 34 people. Recently, eight more of his confessions were confirmed or matched to open cases, the FBI said Tuesday.

But more than half of the confessions remain unconfirmed.

"Little chose to kill marginalized and vulnerable women who were often involved in prostitution and addicted to drugs. Their bodies sometimes went unidentified and their deaths uninvestigated," the FBI previously said.

Because Little, once a boxer, usually knocked out his victims, there weren't obvious signs the victims had been murdered.

The FBI has been unable to link either of these sketches to a victim known by law enforcement. At left is a black female in her late twenties that Little confessed to killing between 1976 and 1979 or in 1993. At right is a white women Little says he killed in 1984 after meeting the victim in Columbus, Ohio. FBI

Another challenge facing investigators is that while Little can describe his victims and how and where he killed them in great detail, he is foggy on dates.

"Little’s uncertain timeline has created a verification challenge ... along with the issues stemming from the victims Little targeted, his methods, and how much he moved around—features of his crimes that begin to explain how he got away with murder for decades," said an FBI statement.

Little pleaded guilty in December to the 1994 strangulation of Denise Christie Brothers in Texas.

He "is in poor health and will likely stay in prison in Texas until his death," the FBI said in November. Their goal "is to identify his victims and provide closure and justice in unsolved cases."

But officials in the places where Little murdered his victims have the choice to bring charges against him, an FBI spokesman has said.