He and his family, as well as Yagami's, were investigated by FBI agent Raye Penber over the leaking of information to Kira. Following Penber's death by Kira, L ordered similar investigations on both families: Kitamura's daughter was even approached by Mogi, posing as L, implying that she was under suspicion of being Kira — while L himself approached the actual suspect, Light Yagami. Kitamura confronted Soichiro Yagami over the issue of his daughter's treatment and was frustrated by Soichiro's insistence that secrecy prevented him from revealing details of the investigation, though Soichiro did admit that it was his own son who was the real suspect.
Kitamura later helped Soichiro out of the Sakura TV studio after he had seized the Kira tapes which the station was broadcasting and which had led to several deaths by Kira. Kitamura sent a riot squad with police buses, shields and face-covering helmets in order to get Soichiro safely out of the building.
However, he was later forced to call off the Kira investigation when politicians were threatened by Kira and bribed by the Yotsuba Group for whom Kira was then working for. He told Soichiro that he had tried to oppose the move but had little choice but to comply while telling the public that the investigation was still ongoing. He did offer Soichiro the right to continue the hunt for Kira, while performing other non-connected police duties, provided that he cut off all ties with L. Soichiro and other members of the Kira Investigation Team decided instead to quit the police force and continue to work with L, eventually exposing Yotsuba and its connection to Kira.
Death Note: How to Read 13 describes Kitamura as not being a "bad guy" in spite of his failings. By the start of the Near-Mello story arc, Soichiro Yagami is the Deputy Director of the NPA but it is not clear if this is along with, or in the place of, Kitamura, whose own fate is unmentioned.