Japanese were constructing submarine base on the Warangoi River according to the U.S. Joint Intelligence Committee. Daily summary dated 14 April 1942

An item reported that the Japanese were constructing submarine base on the Warangoi River, New Britain, Bismarck Archipelago [Australian mandated territory].

Source

Map Room Papers (Roosevelt Administration), 1942 - 1945. U.S. Joint Intelligence Committee. Daily summary No. 125 dated 14 April 1942