FBI investigators have confirmed 34 killings described by Little so far, with many more cases pending.

A 78-year-old convicted serial killer has confessed to committing 90 murders in the United States over several decades, the FBI said this week.

The agency said on Tuesday it is working with state and local agents and detectives to determine if all Samuel Little’s confessions are true.

“Little’s run-ins with the law date back to 1956, and there are clear signs of a dark, violent streak among his many shoplifting, fraud, drug, solicitation, and breaking and entering charges,” the FBI said in a statement. “But law enforcement has only recently begun unravelling the true extent of his crimes.”

Little began confessing to the crimes after he lost an appeal last spring to the life sentences he is serving for strangling three women between 1987 and 1989 in California.

He was convicted of those murders in 2014.

“In all three cases, the women had been beaten and then strangled, their bodies dumped in an alley, a dumpster, and a garage,” the FBI said.

Authorities suspect the unsolved homicides Little said he committed took place between 1970 and 2005 and spanned across at least 14 states stretching from California to Maryland.

FBI investigators have so far confirmed 34 killings described by Little, with many more cases pending.

“Talking with him, you can hear he actually gets excited about describing his homicides and describing how he strangled his victims,” Bernie Nelson, a detective in Prince George’s County, Maryland, told the Washington Post. “He looked you right in your eye and said he couldn’t help himself. He’s a monster.”

The Georgia native has been in custody since 2012, the FBI said.

Little is said to be one of the most prolific serial killers in US history.