How intelligence put Liverpool on the route

Liverpool received a specific warning shortly before the interception, but the surviving files do not identify the original source. The timing and documentary language suggest an informal American contact rather than a formal espionage channel. Once alerted, British planners could use Asama Maru's origin, destination, normal route, and likely speed to estimate where she would pass. The source hypothesis remains tentative; the route analysis provides the firmer part of the reconstruction.