Prosecutors are legal professionals who work for the state and represent the public interest. They have the authority to investigate any crime and indict and try alleged offenders.
Cases they work on are not only sent from police or based on accusations. Prosecutors also perform their own investigations, mainly those involving corruption. A recent example is the arrest and indictment of Takanori Okubo, chief secretary of Democratic Party of Japan President Ichiro Ozawa, by the Special Investigation Department of the Tokyo District Public Prosecutor's Office.
One of the reasons behind the state's 99.9 percent conviction rate is said to be that prosecutors are very conservative and file an accusation when a guilty verdict is certain. But tainting this success rate are the occasional wrongful accusations stemming from forced confessions made during interrogations behind closed doors.