National Police Agency

The National Police Agency (警察庁, Keisatsu-chō), commonly referred to as the NPA is an agency administered by the National Public Safety Commission of the Cabinet Office in the cabinet of Japan, and is the central coordinating agency of the Japanese police system.

Overview
Unlike comparable bodies like the U.S. Federal Bureau of Investigation, the NPA does not have any police officers of its own. Instead, its role is to determine general standards and policies, although in national emergencies or large-scale disasters the agency is authorized to take command of prefectural police forces. Policy for the NPA in turn is set by the National Public Safety Commission.

More information on the NPA can be found here.

The Kira Investigation
After L deduces that Kira is in Japan, L begins working with the NPA via computer. With it decided that searching for Kira could put police officers in great danger, it is announced that any officer working on the Kira case has the option to leave the case with no penalty or cut in paycheck. The NPA becomes the initial source of funding for the Japanese Task Force, the organization in charge of searching for Kira. After the investigation comes to a stand still, the NPA ceases funding of the Task Force.