We had tons of proof of bad actors among those of Japanese ancestry in the U.S. We had broken Japans codes and knew of this.
Magic: The Untold Story of U.S. Intelligence and the Evacuation of Japanese Residents from the West Coast During Ww II
https://www.amazon.com/dp/0960273611
I wasn’t around then, so I don’t know what the thought process was. But it’s remarkable the resources that were marshalled to accomplish such a task. The entire economy was mobilized to a war footing. Rationing - Americans had to turn in empty toothpaste tubes in order to buy a new tube. There is no way the government would have gone through all those contortions - rounding up all those folks, building internment camps in the middle of nowhere, unless somebody thought it was absolutely necessary. No chance.