A former corrections officer has revealed new details about Yoo Young-chul, one of South Korea's most notorious serial killers, including his claims of being haunted by the ghosts of his victims in prison.

Yoo, now 54, was convicted of killing 20 people in Seoul between September 2003 and July 2004. His targets included wealthy seniors and sex workers who provided in-call services. Though he initially confessed to 26 murders, the court confirmed 20 deaths in his 2005 trial.

New information came to light on Thursday's episode of SBS TV’s "Story of the day when the tail was bitten,” where former corrections officer Lee Yoon-hui shared insights from his seven years of weekly interviews with Yoo.

"One day, Yoo came to visit shortly after his imprisonment complaining he couldn't sleep at night," Lee said on the program. "When I asked why, he said victims appear as ghosts every night." Yoo specifically mentioned seeing three to four figures on his prison cell bathroom ceiling.

Lee's account further painted a picture of a man utterly lacking in remorse. When relatives of his victims requested to meet him, Yoo refused any reconciliation. Instead, he made an unsettling offer: to describe their murders in detail.

"That moment I realized we were dealing with a real psychopath," Lee said.

Court records show Yoo had 14 previous convictions for theft, rape and battery before the murders.

After his 2005 death sentence, he was transferred to Seoul Detention Center in Uiwang, south of the capital, where he continues to serve his sentence.

The facility houses several of the country's most infamous killers, including Jeong Du-yeong, sentenced to death in 2001 for 10 murders, and Kang Ho-sun, who received a death sentence in 2009 for killing 10 women.

South Korea hasn't carried out an execution since 1997.