The FBI on Monday verified confessions provided by convicted murderer Samuel Little and certified him as the most prolific known serial killer in U.S. history, according to the bureau.

Authorities have verified 50 of the 93 killings Little, 79, has confessed to and are seeking help to confirm the unverified confessions, the FBI said Monday. Little has confessed to killings ranging from 1970 to 2005.

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“For many years, Samuel Little believed he would not be caught because he thought no one was accounting for his victims,” said Christie Palazzolo, an analyst with the FBI’s Violent Criminal Apprehension Program. “Even though he is already in prison, the FBI believes it is important to seek justice for each victim—to close every case possible.”

Little evaded justice by maintaining a nomadic lifestyle and targeting sex workers and drug addicts, knowing that their murders would be less likely to be investigated. Many of his victims’ deaths were initially believed to be the result of overdoses.

Little was already serving three consecutive life sentences after his conviction for killing three Los Angeles-area women, and he pleaded guilty to killing another in Texas when he made the confession last year. He was extradited to Texas last year after pleading guilty to killing Denise Christie Brothers in 1994 and will likely spend the remainder of his life there.