The FBI said Sunday that Samuel Little, who has confessed to more than 90 murders in the span of 35 years, is the most prolific serial killer in U.S. history after analysts were able to verify 50 confessions so far as part of an ongoing investigation into his past crimes.

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Little, 79, told investigators last year that he strangled 93 people, mostly women, across the country between 1970 and 2005. In a news release Sunday, the FBI said law enforcement were able to verify 50 of those confessions, with several more pending final confirmation. The agency said it believes all 93 confessions are credible, and investigators across the country are working to to piece together the information he provided with unidentified remains and unsolved cases from decades past.

“For many years, Samuel Little believed he would not be caught because he thought no one was accounting for his victims,” ViCAP Crime Analyst Christie Palazzolo said in the release. “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’s death toll surpasses that of Ted Bundy, who confessed to 30 homicides from about 1974 to 1978, and that of John Wayne Gacy, who killed at least 33 boys and young men in the 1970s.

The FBI’s Violent Criminal Apprehension Program (ViCAP) began linking cases to Little about five years ago, the news release said. A Texas Ranger first began about 18 months ago to interview Little from behind bars, prompting him to give scathing on-camera confessions describing how he preyed mostly on women who were prostitutes or lived on the fringes of society.

Little was arrested at a homeless shelter in Louisville, Ky., in 2012 and extradited to California on drug charges, according to The Courier-Journal. DNA samples taken in custody linked him to three unsolved homicide cases dating back to the 1980s. In 2014, he was sentenced to three life sentences without parole.

The FBI provided 30 color portraits, mostly of black women, who Little drew from memory, recalling how he killed each of them. In one of the several on-camera interviews released by the agency, Little spoke about how he strangled one woman to death in 1993 and rolled her body down a slope on a desolate road.

In another video, he described a victim in New Orleans. "She was pretty. Light colored, honey brown skin," he said with a small smile. "She was tall for a woman. Beautiful shape. And, uh, friendly.”

He said it was 1982, and they met in a club. She left with him in his Lincoln, and they parked by a bayou. He recalled: "That's the only one that I ever killed by drowning."

In August, Little pleaded guilty to murdering four women in Ohio. He was convicted in California of three slayings in 2013 and pleaded guilty to another killing last year in Texas. In the Sunday news release, the FBI also provided new information and details about five cases in Florida, Arkansas, Kentucky, Nevada and Louisiana and asked the public for help in identifying the victims.

Authorities in Knox County, Tenn., said Monday that a woman named Martha Cunningham was likely a victim of Little's. The Knoxville News Sentinel reported in December that a cold case investigator with the Knox County Sheriff's Office had identified the victim who Little called "Martha." The Knoxville mother's body was found in a wooded area in eastern Knox County in 1975.

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Cunningham's body was found by a pair of hunters on the afternoon of Jan. 18, 1975. She was bruised and nude from the waist down; her pantyhose and girdle bunched around her knees. Her purse and some of her jewelry were missing. Her body appeared to have been dragged into the woods and dumped behind a pine tree, authorities said at the time.

The Associated Press contributed to this report.