Japanese Intelligence was not so good as it might have been, because no Japanese spies in Alaska had communicated with Tokyo for months. The eight or ten spies had been interned in the States, along with hundreds of innocent Nisei.Garfield gives no source for this claim, and it is what I consider too popular of a history -- only some items are footnoted. The fact that Garfield gives a rather exact number "eight or ten spies" does make me suspect that he had some authoritative source for this, perhaps speaking off the record because the information was still classified at the time. Otherwise I suspect that this would have "some" or similar vague statement about Japanese spies in Alaska.
Michelle Malkin put herself in harm's way several years ago pointing out that information declassified in the 1990s showed that the U.S. government knew that there were some Japanese spies among the Americans of Japanese ancestry in the U.S. At least, Japanese diplomats referred to them working in defense plants in dispatches back home, and the U.S. had broken the Japanese diplomatic code before the war.
The number was doubtless tiny, compared to the 100,000 or more Japanese citizens or American citizens of Japanese ancestry who were interned as a security risk. It would have been better if the U.S. government had put more energy into identifying those who were disloyal, instead of locking up everyone -- the vast majority of whom proved their loyalty in spite of this mistreatment. (Many proved their loyalty in the ultimate way, in the 442nd Regimental Combat Team.)
It is also true that there were other motivations for the internment, including a long history of racism driven by unfair practices of Japanese farmers (such as hard work and intense agricultural practices brought from Japan), and more immediate hatred because of Pearl Harbor. It is possible that these other motivations were actually the larger motivation, especially by the time the internment actually took place. But to pretend that there was no national security concerns that played a part in the decision is just dishonest.
UPDATE: unfair practices "such as hard work and intense agricultural practices" is sarcasm. Have we really so fully reached the Obamanation that I have to be explicit about this? Oh what a tragedy.