When he heard the "East wind, rain" code phrase on the short wave radio bringing the news from Tokyo to signal that an attack against the United States was to proceed, Yoshikawa destroyed all evidence of his activities. 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.