From Saipan and Guam in the Mariana Islands, an invasion fleet carrying 5,000 Japanese troops departed for Midway Atoll under Rear Admiral Raizo Tanaka. In the northern Pacific Ocean, Japanese submarine I-25 launched her floatplane for a reconnaissance mission over Kodiak Island, US Territory of Alaska, which spotted a US cruiser and two destroyers. To the west, still in the Aleutian Islands, I-19 was preparing to launch her floatplane when lookouts spotted an American aircraft; the submarine dove for cover, destroying the floatplane in the process. In Japan, the naval leadership affected a radio encryption coding change for all fleets and bases. At Pearl Harbor, US Territory of Hawaii, Joseph Finnegan and Wesley Wright, two intelligence officers working under Joseph Rochefort, broke the Japanese encryption used to secure operational dates after working on it for the entire previous night; with this new knowledge they determined that the Japanese were going to raid Dutch Harbor in Alaska on 3 Jun 1942 and to attack Midway Atoll on 4 Jun 1942. Finally, Rochefort visited Chester Nimitz's office to help him convince other admirals and generals of their belief that Midway was soon to be a Japanese target.
