Police said Monday they have obtained an arrest warrant for a man in his 20s on suspicion of aiding the gangster who has admitted shooting Nagasaki Mayor Itcho Ito.
The Nagasaki Prefectural Police said the suspect, whose name was not provided, was a driver for mobster Tetsuya Shiroo, 59, who was arrested at the scene of the assassination.
The driver is suspected of monitoring the mayor's movements at places other than his election campaign office. The shooting took place on the street outside the office.
The suspect has been placed on a police wanted list.
On Sunday, police arrested 60-year-old Hiromi Ogawa, who they said has admitted giving Shiroo a ride to the scene of the attack outside JR Nagasaki Station shortly before Ito, 61, was gunned down while campaigning for re-election in the April 22 mayoral poll.
Prior to the assassination, Shiroo, who is said to have borne a personal grudge against the city, had forwarded a letter to TV Asahi Corp. that read, "I cannot forgive Mayor Itcho Ito."
The broadcaster received the letter April 17, the day of the shooting.
Police suspect Ogawa actually penned the letter on Shiroo's behalf and will investigate whether Ogawa had prior knowledge of his intention to kill Ito.
Ito was shot twice in the back at point-blank range and died early April 18.
Police arrested Shiroo, acting leader of the Suishin-kai underworld affiliate of Japan's biggest organized crime syndicate, the Yamaguchi-gumi, after witnesses wrestled him to the ground and seized a revolver from him.