I suppose because when they pick you up and toss you in prison they check your status.....
You're welcome.
susie
Thanks for missing the point completely -- the point was not addressing the number of illegals in prison, but the number of illegals outside prison, that were stated to be "unknown." If you don't know how many total illegals there are, you cannot expect to make any serious statistical inferences about those in prison, as a proportion of the "illegal" population.