Samuel Little is being called the most prolific serial killer in U.S. history after authorities verified more than half of the 93 murders he's confessed to during a 35-year span.

The 79-year-old inmate was already serving three consecutive life sentences in California when he confessed to another 90 killings last year. On Monday, the FBI confirmed that its investigators verified 50 of those deaths and are seeking help to confirm the rest of the unmatched confessions.

The FBI’s Violent Criminal Apprehension Program, also known as ViCAP, began connecting Little to unsolved murders after a Texas Ranger tied the inmate to a 1994 cold case in the city of Odessa.

Not long after, Little confessed to strangling 93 women across the country between 1970 and 2005.

Gary Ridgway, dubbed the Green River Killer, was convicted of 49 murders and confessed to about 20 more. Ted Bundy and John Wayne Gacy each murdered upwards of 30 people, but Bundy was suspected of more.

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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,” Christie Palazzolo, a ViCAP analyst, said. “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, who is in failing health, was extradited to Texas last year where he entered a guilty plea in December for the 1994 death of Denise Christie Brothers. He's likely to spend the rest of his life in prison in the Lone Star State.

Samuel Little appears at Superior Court in Los Angeles on March 4, 2013 Damian Dovarganes / AP file

A number of factors helped Little escape justice for decades, including the former boxer's nomadic lifestyle. Little moved from city to city targeting vulnerable women, such as sex workers or drug addicts, who sometimes went unidentified and their deaths weren't investigated, according to the FBI.

Limitations of DNA evidence also played a role in how long it took for authorities to pin down Little's killing spree.

Authorities were finally able to convict Little after he was arrested in Kentucky in 2012 and extradited to California to face a narcotics charge. The Los Angeles Police Department tied Little's DNA to three murders between 1987 and 1989.

Though Little maintained his innocence at the time, he was convicted in 2014 to three life sentences.

Texas Ranger James Holland, who flew to California to speak to Little, is credited with getting the serial killer to confess to the other 90 murders. After Little was extradited to Odessa, Holland conducted almost daily interviews to create the most accurate accounting possible of Little’s crimes, according to the FBI.

As the FBI works to confirm Little's 40 other murders, investigators have requested that anyone with information contact the bureau at 1-800-CALL-FBI.