There is no definitive U.S. Muslim population census.
However, there are multiple estimates that span from 1.6 million, 2.8. million, and more probably inflated estimates reaching upward toward 6 million U.S. Muslim residents.
The correct question is 'How many active Radical Muslims reside within U.S. cities, that can bring the U.S. another terrorist attack'?
Now that is the real question --- isn't it!
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The Seattle Times: Nation & World: Muslim immigration has bounced back Muslim Statistics for the U.S. / Number of Muslims in America