Xerodin

The second sentence of the Wikipedia article literally says about 2/3 were American citizens.

The section on ‘Exclusion, removal, and detention’ says “[s]omewhere between 110,000 and 120,000 people of Japanese ancestry were subject to this mass exclusion program, of whom about 80,000 Nisei (second generation) and Sansei (third generation) were U.S. citizens.”

So yes, second and third generation Japanese Americans, natural born citizens, were held in American concentration camps.

Unrelated, but I fucking love Cross Code.