Feb 19, 1942: FDR issued the execute order to analyze the Japanese population (both American citizens and immigrants) to see if they posed a threat, particularly on the west coast. Keep in mind that German spies during WW1 detonated Black Tom Island in New York Harbor in 1916. The EO specifically allowed areas to be designated "military areas" that required every possible precaution against espionage and sabotage.
Feb 23, 1942: A Japanese sub attacked Inglewood, California, a fuel refinery. No real damage was done, but folks freaked out. Particularly with the press (the drive by media wants you to forget the part they played in this) did reports of signal lights between people on land and the sub. THAT's when the major internments and relocations began.
https://www.nytimes.com/1983/05/22/us/1941-cables-boasted-of-japanese-american-spying.html
… Mr. Lowman, who in the 1970’s worked on the declassification and publication of the decoded Japanese cables, said that Roosevelt, Secretary of War Henry L. Stimson and a handful of other officials were on the limited distribution list of the cable traffic.
‘’Anyone reading this flow of messages during 1941 could easily conclude that thousands of resident Japanese were being organized into subversive organizations,’’ Mr. Lowman said. ‘’Today we know that the Japanese Government misjudged the loyalty of Japanese Americans completely. But at that time no one knew for certain.’’…