Police on Thursday said Lee Chun-jae's confession to an additional murder which authorities previously concluded was perpetrated by a copycat killer contained significant parts that lend it credibility.

Lee, 56, recently admitted that he was behind all ten of the Hwaseong serial murders carried out between 1986 and 1991.

Gyeonggi Nambu Provincial Police Agency said in a press briefing that it is trying to draw definitive testimony from Lee, who is currently serving life for the rape and murder of his sister-in-law in 1994.

When asked whether Lee's confession was detailed, police said his confession contained parts of significance, and added that they were working to draw out testimony only the real perpetrator would know. 

Lee’s confession, which came after authorities linked his DNA to some of the Hwaseong murders last month, has stirred controversy as a then 22-year-old mechanic, surnamed Yoon, had been tried and convicted for the eighth murder.

Yoon claimed he was innocent during two appeals, but nonetheless served 20 years in jail. After his release in 2009, Yoon said he was tortured into giving a false confession.

Police plan to probe the detectives who investigated Yoon, all of whom have apparently retired.