American security was tight in the days leading to the Pearl Harbor attack
On March 27, 1941, the Japanese liner Nitta Maru arrived in Honolulu carrying a new Japanese Consul-General, Nagao Kita, and his vice-consul, Tadashi Morimura. Both quickly became involved in espionage activities, tasked with obtaining information regarding the American fleet movements to and from Pearl Harbor. Morimura obtained rented quarters near Pearl City, in the hills overlooking the harbor, and studiously avoided the large Japanese-American community in the area. He preferred to take long walks alone in the hills overlooking the harbor, and around other military facilities in the region. He also cultivated a relationship with Bernard Kuehn, an agent working for the German Abwehr. Unknown to the American intelligence community at the time was Morimura was actually a former Japanese naval officer and intelligence agent of the name Takeo Yoshikawa.
Yoshikawa collected intelligence regarding ship movements, troop strengths, anchorages, schedules, and whatever else he could learn from observation and listening. These he transmitted via the Japanese Purple Code at the consulate to his handlers at the Japanese Foreign Ministry. Though the United States had by then broken the Purple Code, and intercepted his reports, no action was taken. The FBI and military intelligence considered most of the messages from the consulate to be commercial in nature, and thus of little interest to the military. Yoshikawa sent information routinely, much of which was used by Japanese commander Yamamoto in finalizing the preparations for the attack. He then destroyed all evidence of his activities. Following the attack, Yoshikawa and other Japanese diplomats were seized by the FBI, but by then no evidence of his espionage activities remained. He was returned to Japan in a diplomatic exchange, never charged as a spy.