The police in Mie Prefecture located the computer where the threats apparently came from and brought the 27 year-old man who owned it into police custody. A week later he was released. The man, it seems, was apparently the victim of a computer virus. And it seems he was not the virus' first victim.
According to police reports via NHK, the 27 year-old's computer was infected with a type of computer virus previously unseen in Japan. The virus enabled a third party the ability to control the host computer.
This is apparently what happened to animator Masaki Kitamura, who's worked on a whole host of projects, such as Yu-Gi-Oh! (storyboard), Zone of the Enders: Idolo, Tiger & Bunny (episode director), and Mobile Suit Gundam 00 (storyboard, assistant director, and episode director).
"I will ram into pedestrian paradise, and after stabbing indiscriminately with a knife, I will commit suicide.
In July, a message appeared on the Osaka city inquiry page, which read, "I will commit a massacre in Otaroad next Sunday. I will ram into pedestrian paradise, and after stabbing indiscriminately with a knife, I will commit suicide." Otaroad is in Nipponbashi, Osaka's geek district. It's an area with heavy foot traffic—a pedestrian paradise, if you will. The threat seemed similar to the 2008 attack in Akihabara in which seven people ended up dead and ten were injured.
This time, Osaka police traced the Otaroad threat and discovered it was from a high-speed wireless LAN device owned by Kitamura, who at the time said, "I have zero recollection of doing this." Japanese police have released Kitamura, after supposedly detecting the same virus that was discovered in the 27 year-old's computer. The assertion is now that his computer could also have been controlled remotely.
As previously mentioned, news reports state that this virus is new to Japan. Authorities are now investigating how both of these computers were infected and how the virus spread, as well as continuing to look for the culprit.
大阪市に殺人予告、第三者の疑い＝ウイルス感染で脅迫文言―演出家、無実か [Yahoo! Japan]
遠隔操作ウイルス 三重でも男性釈放 [NHK]