Police arrested Shiroo on suspicion of the murder after he was detained by Itoh's entourage and passersby following the shooting. Shiroo was a leader of the Suishin-kai, a Nagasaki-based yakuza group affiliated with the Yamaguchi-gumi, the largest yakuza organization in Japan. Shiroo had shot Itoh with a Smith & Wesson Model 36 and was found with 20 bullets in his possession upon arrest. Several theories exist for Shiroo's motive to shoot Itoh, with the main theory being a personal grudge against Itoh and the city government for an incident in 2003. Shiroo filed an insurance claim for damage to his car when he drove it into a hole dug for public works, but the city refused to compensate him. Shiroo had mailed a letter to TV Asahi about the issue, and it was public knowledge before the shooting. Additionally, Shiroo was also reportedly angry that a construction company linked to the Suishin-kai had been denied a contract by the city government. Itoh's assassination had not been approved by the Yamaguchi-gumi and led to greater public scrutiny of yakuza activity. The Suishin-kai's offices were raided by police for evidence, and the gang announced their intention to voluntarily disband days later.