I mean the title pretty much says it all, but I remember watching Memories of Murder back in the day, and this film seems like it's related--yet I haven't found anything which definitely states that this is the case.
I also got the feeling that this film was based on the same case. Both films involve a series of about ten murders taking place between 1986 and 1990/91, which is obviously based on the real-life case. So I see this film as sort of a fictionalization of the events transpiring afterwards, with some liberties taken for the sake of entertainment, of course. I mean, it can't just be a coincidence that this film describes the exact same murders that actually took place. They had to have drawn at least some inspiration from the actual case, considering how notorious it was.
It's kind of an "unofficial" sequel. If you notice the flashback to the murder of the girl in the drain pipe you can see it was inspired by the same real life case.