The Silver Case

1999 - the "24 Wards". A string of mysterious serial killings plagues the city. The detectives of the 24 Wards Heinous Crimes Unit have their eyes on one man: Kamui Uehara, legendary serial killer and assassin of a number of government officials 20 years prior in the now-famous "Silver Case". However, nobody knows who - or what - Kamui really is. Has he really returned? Who is this infamous serial killer?

[From Steam]

The game also has an adventure game sequel, "Flower, Sun, and Rain".