South Korea's most notorious serial killer murdered 15 women, police said Thursday, as they apologised for a botched, decades-long investigation that saw an innocent man jailed for 20 years.

Lee Chun-jae raped and murdered 10 women in rural parts of Hwaseong, south of Seoul, over a five-year period from 1986, police said.

At the time a record number of police officers for a single case were mobilised to try to find the killer, investigating some 21,000 individuals and comparing the fingerprints of around 20,000 more without success.

The inquiry inspired Oscar-winning South Korean filmmaker Bong Joon-ho's 2003 box office hit "Memories of Murder".

Lee, 57, was only identified as a suspect last year -- more than 30 years after killing his first victim -- using the latest forensic techniques to retrieve DNA from long-past crimes.

Under questioning, he had admitted all 10 Hwaseong murders, as well as four others including an eight-year-old girl, police said, adding he also raped nine other women.

"His psychopathic tendencies were evident, as he was unable to empathise with the victims' pain and sufferings at all and continuously showed off his crimes," said provincial police chief Bae Yong-ju.

Bae bowed before the cameras as he apologised to the victims' families and those who had been falsely accused as suspects, as well as a man who was wrongfully convicted of one of the killings and served 20 years in jail.

Lee is currently serving a life sentence for raping and murdering his sister-in-law in 1994, but will not be prosecuted for his other crimes as the statute of limitations has expired, police said.

Thursday's announcement confirms that a man surnamed Yoon -- who was convicted in 1989 of raping and killing a teenage girl among the Hwaseong victims -- was wrongfully jailed.

Yoon was released on parole in 2009 after serving 20 years in prison, and filed for a retrial last year after police launched their investigation into Lee.

Reports say at least four individuals took their own lives in the 1990s after being investigated -- and allegedly abused --  by police as possible suspects in the Hwaseong murders, whose victims were aged 14 to 71.

"I bow my head down and offer apologies to the victims of Lee Chun-jae's crimes, the surviving families and everyone who suffered damage due to the police investigation, including Mr Yoon," said Bae.