 Japanese radio intercepts did notice an increase in both American submarine activity and message traffic . This information was in Yamamoto 's hands prior to the battle . Japanese plans were not changed ; Yamamoto , at sea in Yamato , assumed Nagumo had received the same signal from Tokyo , and did not communicate with him by radio , so as not to reveal his position . These messages were , contrary to earlier historical accounts , also received by Nagumo before the battle began . For reasons which remain unclear , Nagumo did not alter his plans or take additional precautions . 
