Why couldn't Japan crack the Navajo Code in WW2, when they captured a Navajo native speaker?

Question: Why couldn't Japan crack the Navajo Code in WW2, when they captured a Navajo native speaker?

Because it wasn't enough to just speak Navajo, they still had to know the code.

So let's say in the code a turtle signified a company of soldiers and up the hill signified heading to the south coast of the island. It would take a Navajo speaker to tell you 3 turtles were headed up the hill. But if you didn't know what that meant, you had no hope in understanding the code.

The code was spoken over wired telephone lines which the Japanese tapped into. The messages might have been broken if the given Navajo speakers were situationally relevant to the message. But being POW's, they were often given the messages to translate removed from the battlefield both by time and distance. This meant they couldn't compare the messages to actual events on the battlefield to try to decipher them. They could not just read, 3 turtles crawled up the hill, and see three companies of men began to head down to the south coast, and associate the two events.

From Comments:

@Oleg Lobachev How hard is it to learn the needed amount of Navaho anyway? An intelligence officer could do this and then observe the field... –

It's basically impossible to learn in a few months in order to be helpful to the Japanese, especially in a wartime Japan, removed from any resources. What resources would a Japanese code breaker have to learn a language if the population who speaks that language is behind enemy lines and few language texts exist?

PBS
Although Navajo is the most-spoken Native American language in the U.S., it is rarely spoken outside of the Navajo reservation. An unwritten language without a traditional alphabet or symbols [a written language was developed prior to WWII], Navajo's extreme complexity of syntax makes it unintelligible to anyone without extensive exposure and training.

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from Kentaro
Sorry I reversed my upvote. The reason is your above claim without any proof. – (This claim without any proof)

@JMS My understanding is that the Japanese knew the Marines were using a Navaho Indians as code talkers.

My apology, the comments are too tight a space to give sources, so often I don't even try. I say the Japanese knew the Marines were using Navajo Indians as code talkers, because they sought out a Navajo POW and enlisted him to help break the code. Thus the Japanese knew they were dealing with not only an American Indian code, but specifically one based on the Navajo language.

Joe Kieyoomia was a Navajo soldier from New Mexico, who was not a code talker. He was captured by the Japanese when the Philippines fell at the beginning of the War. The Japanese tried unsuccessfully to put him to work trying to crack the Navajo code.

Joe Kieyoomia
The Japanese tried unsuccessfully to have him decode messages in the "Navajo Code" used by the United States Marine Corps, but although Kieyoomia understood Navajo, the messages sounded like nonsense to him because even though the code was based on the Navajo language, it was decipherable only by individuals specifically trained in its usage.

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@Swansea
@JMS Why weren't the Japanese deliver intercepts to the Navajo POWs and back to Japanese intelligence in a timely manner, like you wrote? To use the example in my quote, why didn't the Japanese report, after the US called "Apache support", everything that followed including helicopter gunships to the Navajo POWs?

• 1. These battlefields on Pacific Islands weren't small localized affairs, they involved tens of thousands of men, Battle of Okinawa utilized 180,000 American combat troops. There were no "Apache helicopters" in WWII, but how could the Japanese know what any given message would refer to when their enemy was operating across such a scale?
• 2. The code conveyed by the Navajo was professionally done. It wasn't as easy as just listening for key words and trying to match them up to what was happening. Think of a code done in English. It wouldn't be much of a code if once hearing it a few times the other fellow knew what you were speaking of.
• 3. I don't believe the Navajo POW in question Joe Kieyoomia ever left his POW camp in Nagasaki. It wasn't practical to send him to Islands under invasion. The US had literally hundreds of Navajo code talkers, The Japanese weren't just looking for one guy, they needed the ability to train hundreds of intelligence officers in this code in order to capitalize on breaking it. Of coarse though they never broke it.
• 4. Japanese intercepts were from radio and hard wired messages. Both had challenges in utilizing POW's proximal to the battle field. Both necessitated a clandestine component which would prohibit also having those sources being entirely aware of occurrences on a battle front which could be many miles across. Wire taps had to be conducted behind American lines, it would have been difficult to bring a Navajo fluent POW on such a mission; much less transport him to remote locations under siege.

from Kentaro @So, I need the source! You don't have to hurry, your answer is not bad, so I'm waiting for your effort. Anyway, thanks

( on how the JIA associated the Navajo Language with the unbreakable code. )

@Kentaro, It seems like interrogations of the many Marine POW's the Japanese captured would be the obvious source. I'm not stating that as a fact, although it probable is how they found out. I did find this source which claims a Japanese interrogator Goon pieced it together upon interrogating Joe Kieyoomia. That doesn't seem reasonable that this Translator thousands of miles removed from the battle field in Nagasaki would piece this together. I think captured POW's who would certainly have been questioned vigorously on knowledge of this code would be the more reasonable source. Then Goon got involved when the Japanese searched their camps for a native Navajo speaker. That's just my hypothesis. The Japanese knew about Joe Kieyoomia because when they captured him in the Philippines they mistook him for an ethnically Japanese person. It took some time for him to convince them he was Navajo. Here is that source.

Code Talker: The First and Only Memoir By One of the Original Navajo Code Talkers
Despite the United States' insistence upon secrecy, the Japanese somehow learned that the unbreakable code being utilized by the Americans had something to do with the Navajo language. No one knows exactly how or when this information was obtained, but it has been hypothesized that a Japanese translator with the surname Goon first associated the Navajo language with the unbreakable code while participating in the interrogation of Joe Kieyoomia. Kieyoomia, a Navajo man who has survived the Bataan Death March, was questioned by Goon and tortured by his Japanese captors in their attempt to force him to crack the code. ....

Several Navajo prisoners reported, postwar that the Japanese had tried to get them to figure out the marine's code. None of these captives were code talkers, and none shed any light on the complicated secret language.