Surveilling peacetime Pearl Harbor military activities was an IJN Ensign by the name of Takeo Yoshikawa:
https://en.m.wikipedia.org/wiki/Takeo_Yoshikawa
Quote:
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"When the FBI picked him up on the day of the attack, there was no incriminating evidence of his espionage. He eventually returned to Japan in August 1942 in a diplomat prisoner exchange. It was not known for some time that he was the chief Japanese agent in Hawaii."
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His messages to his homeland were encrypted; however, the US was capable of reading them.
Unfortunately, "manpower shortage" had left the messages unread.
Waikiki Beach as you've never seen it:
https://pacificwrecks.com/location/h...ki/wakkii.html
