Between 18 and 20 September 2004, four members of the Kitamura-gumi (北村組), a yakuza gang based in Ōmuta, Fukuoka, Japan, affiliated with the Dōjin-kai crime syndicate, killed four people. Mami Kitamura had borrowed money from a 58-year-old woman, Sayoko Takami. On 18 September 2004, Mami, her husband, and her two sons strangled Sayoko and shot her 18-year-old son Tatsuyuki and his 17-year-old friend Junichi Hara. The perpetrators placed the three victims' bodies in a car, which they then dumped into the Suwa River in Ōmuta.
On 20 September 2004, the same group strangled Sayoko's 15-year-old son, Jōji. His half-naked body was found the following day, 21 September. When police arrested Mami on 22 September, she confessed to the killing of the other three victims, and police subsequently recovered the car containing their bodies from the Suwa River.
The other participants named in the killings were Mami's husband, Jitsuo Kitamura, the leader of the Kitamura-gumi; Takashi Kitamura, her son from a previous marriage; and Takahiro Kitamura, her second son. Takashi and Takahiro were both former sumo wrestlers, known respectively as Kyokuryūjin and Miikezan, under the family name Ishibashi. As police closed in on the group, Jitsuo attempted to kill himself with a handgun. Takashi escaped from police custody but was recaptured. Jitsuo maintained that he had committed the murders alone, but police regarded Mami as the primary offender.
The defendants were reported as disruptive during their trials. On 17 October 2006, Mami and Takahiro were sentenced to death. Jitsuo and Takashi received death sentences on 28 February 2007. On 25 December 2007, the Fukuoka High Court upheld the original sentences for Mami and Takahiro; Takahiro reportedly shouted "Merry Christmas!" in court following the ruling. On 27 March 2008, the Fukuoka High Court also upheld the original sentences for Jitsuo and Takashi.
Separately, Takashi Kitamura had a prior conviction for a June 2000 incident in which he and six associates beat an 18-year-old boy with a bokken (wooden sword) before he was dropped into an irrigation canal and drowned; Takashi was sentenced to three years and six months in prison for that offense.