Charged with the kidnapping and murdering four young girls in Tokyo and the Saitama Prefecture from 1988 to 1989, Tsutomu told the Tokyo District Court he did not, "have much interest in his trial." At the outset of the hearings in 1990, Miyazaki admitted he abducted and killed the four girls but said he felt "as if I had done it in a dream." He later said in court that he "did not remember" the deposition statement in which he owned up to the charges.
Court hearings were suspended on March 1993 when lawyers demanded a second psychiatric test for Miyazaki. The first psychiatric tests showed that his mental condition, despite signs of personality disorder, was sufficient enough to be held responsible for the crimes. But a second test contradicted the initial one, suggesting that he was either suffering from multiple personality disorder or schizophrenia.
According to the court, Miyazaki kidnapped a 4-year-old girl in Saitama Prefecture and strangled her in a forest in suburban Tokyo in August 1988. About five months later, he burned the body near his home. The he placed the ashes in a cardboard box that he left in front of the dead girl's home.
In October the same year, he abducted a 7-year-old girl while she was walking along a street in Saitama Prefecture and took her to woods in suburban Tokyo and strangled her. In December 1988, Miyazaki approached another 4-year-old girl in Saitama Prefecture, strangled her in his car in a parking lot and abandoned her body in a nearby wooded area, according to the court. In June 1989, he abducted a 5-year-old girl in Tokyo and also strangled her in his car. He later mutilated her body and abandoned parts of the corpse in wooded areas of Saitama and Tokyo prefectures.
Miyazaki was arrested in July 1989, in Hachioji, Tokyo, on charges of trying to molest another girl after he was caught by the girl's father. He confessed during questioning to the four slayings. After his arrest, investigators found and confiscated a collection of 5,800 animated and other videos at his home. Among the videos was one with about five minutes of footage of one of the mutilated victims, as well as a video tape from the "Guinea Pig" series in which a man kidnaps a nurse, renders her unconscious by injection and cuts off her hands, head and feet. Miyazaki later said he had felt "uncomfortable" with adult women and turned to children. An equal oportunity psychopath, he added that he was attracted to necrophilia.