Samuel Little described his crimes in painstaking detail, sometimes smiling or laughing at the memory. Over 18 months of confession to a Texas Ranger inside the prison where he's already serving a life sentence, the gray-haired, 79-year-old man admitted to 93 murders spanning 19 states and 35 years.

With 50 of those cases verified, the FBI announced that Little, whose gruesome crimes went undetected for decades, is the most prolific serial killer in U.S. history. Now, investigators are seeking the public's help in identifying the rest of the people he's confessed to having strangled between 1970 and 2005. They believe all of his confessions to be credible.

On Sunday, Oct. 6, the agency released a trove of information on five unmatched cases, including video clips of Little recounting meeting the victims and dumping their bodies. He described their appearances - "she was pretty . . . she had a beautiful body on her," he said of a woman he killed in New Orleans - as well as the fields, canals and trash heaps where he left them. He also drew portraits of their faces.

The FBI says Little targeted mostly vulnerable women. Some were sex workers or drug users. Others were transgender. Their deaths may not have attracted much attention, and authorities often cited drug overdoses, accidents or natural causes as responsible. In some of the cases, the bodies were never found.

"For many years, Samuel Little believed he would not be caught because he thought no one was accounting for his victims," FBI crime analyst Christie Palazzolo said in a statement. "Even though he is already in prison, the FBI believes it is important to seek justice for each victim - to close every case possible."

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Until now, Gary Ridgway was thought to be the country's most prolific serial killer. Nicknamed the Green River Killer, he was convicted of 49 murders. Investigators say they have not found reason to doubt Little's claims of being responsible for 93 slayings. On top of the 50 confessions verified so far, the FBI said, there are "many more pending."

Little, who may have been born in jail while his mother was incarcerated, was raised by his grandmother in Ohio, according to the New York Times. A high school dropout and onetime competitive boxer, he spent much of his life crisscrossing the country, getting arrested repeatedly for crimes such as shoplifting, armed robbery, kidnapping and rape. In the 1980s, he was charged with killing women in Mississippi and Florida, the FBI said. But he escaped indictment in Mississippi and conviction in Florida.

It wasn't until 2014 that Little was convicted of murder. He had been arrested at a Kentucky homeless shelter and extradited to California on a drug charge. While he was in custody, Los Angeles police detectives obtained a DNA match to three unsolved homicides from the late 1980s, in which women had been beaten and strangled. Little received three life sentences for those crimes.

The FBI began looking into his background and discovered "an alarming pattern and compelling links to many more murders." But Little insisted he was innocent - until Texas Ranger James Holland managed to win his trust. After that, the FBI said, "a breathtaking number" of confessions came pouring out. Little would stroke his face and look up as he pictured a victim, and then rattle off a detailed description, Holland told "60 Minutes."

"And you can tell he has this revolving carousel of victims, and it's just spinning, and he's waiting for it to stop at the one that he wants to talk about," the Texas Ranger said in a segment that aired Sunday.

In the videos the FBI released over the weekend, Little appeared to have no remorse and was excited as he talked about the killings. Wearing a gray skullcap and blue prison scrubs, he described a woman he thought was named Ruth. "Oh man, I loved her," he said, and then described leaving her body in a corn field in north Little Rock.

He said he gave a transgender woman named Marianne a ride to her apartment in Miami, where one of her roommates asked them to buy a can of shaving cream. "This opportunity popped up," Little said, smiling, and explained that he drove her out to the Everglades instead.

Despite the many details Little recalls, he is not always clear on the dates of his crimes. The FBI said that - along with the victims he targeted, the methods he used and the frequency of his moves around the country - has created challenges in identifying victims.

Those aspects also begin to explain, the FBI said, "how he got away with murder for decades."

This article was written by Brittany Shammas, a reporter for The Washington Post.