The high school graduation photo of Lee Chun-jae, left, and a facial composite of the Hwaseong serial killer.
Seoul, South Korea CNN  — 

South Korean police have admitted using violence to extract a false confession from a man who spent 20 years in prison for the rape and murder of a 13-year-old.

Yoon – whose full name is not being published due to a South Korean law that protects the privacy of suspects and criminals – was the only person ever convicted of the so-called Hwaseong murders, one of South Korea’s most infamous serial killing cases. He was released in 2008 after spending 20 years in prison for the rape and murder of a 13-year-old in 1988, with police suspecting that the case was a copycat killing.

On Thursday, Gyeonggi Nambu Provicial Police Agency chief Bae Yong-ju said authorities had concluded 57-year-old Lee Chun-jae was responsible for all 10 killings that took place between 1986 and 1991 in Hwaseong, then a rural area near the South Korean capital Seoul.

The announcement followed a police probe launched last year into the cold case after new DNA evidence connected Lee with at least some of the killings. Following that, Lee confessed to all 10 of the Hwaseong murders, four other murders, and 34 rapes, police said.

On Thursday, Bae admitted that during the initial investigation in 1989, police assaulted Yoon and coerced him into making a false confession. An official document had noted that a witness was present during Yoon’s confession – but on Thursday, Bae said that was not the case.

“We bow down and apologize to all victims of the crimes of Lee Chun-jae, families of victims, and victims of police investigations, including Yoon,” Bae said Thursday, noting others had suffered from “police malpractice” during the initial Hwaseong investigation.

Kim Chil-joon, an attorney who defended other Hwaseong suspects, has previously told CNN that many people were abused during the investigation.

Bae said seven police officers and a prosecutor involved in the initial investigation into Yoon had been formally investigated for abuse of power and unlawful detention. Under South Korean law, the statute of limitations has run out on the cases, meaning those officers cannot be indicted on any charges.

Police have also passed 14 murders and nine rapes allegedly committed by Lee to the prosecutors’ office. However, Lee cannot be prosecuted for any of the cases as the statute of limitations on those has expired.

Police authorities investigating the crime scene of the Hwaseong serial killings in Gyeonggi Province.

Lee is already serving a life sentence for the 1994 rape and murder of his sister-in-law, according to Daejeon court officials and South Korea’s Justice Ministry.

On Friday, Yoon told CNN he was relieved to hear police say Lee was behind the Hwaseong killings, including the one he went to prison for. A retrial of his case is underway – a rarity in South Korea, where only a tiny fraction of applications for retrials are accepted. If Yoon’s conviction is overturned, he will be able to apply for compensation.

Yoon said he would feel better once the retrial was over.

“I feel so frustrated about those long years (without justice),” he said. “If the police who interrogated me made an apology, I’d feel better.

“More than owing me an apology, I think the police owe an apology to the people of South Korea. Can you imagine how many people could have been treated unfairly or wrongfully accused by police in the past years?”