Between 1986 and 1994, Lee murdered fifteen women and girls in addition to committing numerous sexual assaults, predominantly in Hwaseong, Gyeonggi Province, and the surrounding areas.

[1] Lee was sentenced to life imprisonment with the possibility of parole after twenty years for killing his sister-in-law in 1994, but despite DNA evidence and his confession to the other murders in 2019, he could not be prosecuted for them because the statute of limitations had expired.

[3][4] After graduating from high school in February 1983, Lee joined the Republic of Korea Army and served as a tank driver; he was discharged in January 1986.

The following year, he was employed as a crane driver for a company in Cheongwon, North Chungcheong Province, which he ultimately quit in March 1993.

[6] He was sentenced by the Suwon District Court to one year and six months' imprisonment in the first trial in February 1990 for the charges of robbery and violence.

[6] After the first trial, Lee filed an appeal, claiming that he was beaten by an unknown young man and entered the victim's house while being chased.

[8] The victims, all women, were found bound, gagged, raped, and in most cases strangled to death with their own clothes, such as pantyhose or socks.

[6] Lee was arrested a few days later, on January 18,[13] after repeated questioning where he later asked, "How many years do you serve in prison for rape and murder?

"[6] Lee denied any responsibility and the court overturned his confession, stating that he made a false statement because of police coercion.

The Supreme Court of Korea reviewed the case in 1995 and Lee's death sentence was reduced to life imprisonment,[14] with the possibility of parole after 20 years.

[20] According to the victims, the culprit at the time of the incident was a thin-framed man in his mid-20s, with a height of 165 to 170 centimeters, short cut sporty-type hair, no double eyelids, and a sharp nose.

[28] The murder of a female college student in Hwaseong in 2004 also sparked renewed interest and fears that a serial killer had returned.

[44] Police expressed that Lee had weak self-esteem due to his introverted personality but experienced a sense of accomplishment and self-reliance for the first time in his mandatory military service, which led him to commit sex crimes to express his frustration caused by his monotonous life following his discharge from the military.

[45] The provincial police chief stated that Lee displayed psychopathic tendencies by being unable to empathize with the victims' pain and suffering and continuously showing off his crimes.

[48] Yoon was sentenced to life in prison, but appealed the ruling at the time, alleging that police coerced him into giving false confessions through torture.

[49] Two days later, police announced that they had reached a provisional conclusion that Lee was responsible for the murder for which Yoon was convicted.

The final court hearing was held on November 2, 2020, where Lee stood as a witness confessing to the murder and describing the crime scene.