There has been much gnashing of teeth about some candidates' consideration of establishing a list of Muslims in the U.S. Before you get entirely wee-wee'd up about this invasion of privacy, take a look at The Washington Post's Philip Bump article appearing in The Boston Globe.
According to Bump, this info is already in existence, compiled by the private sector. A competent data miner can purchase and cross-reference together a list that would be, if not perfect, darn close to it.
My guess: the FBI has already has built this list, perhaps with the aid of the NSA, and is quietly laughing up their sleeve at all the fuss. Given the ease and relevance, if they haven't done so somebody should be fired or, failing that, sent to monitor government fraud in Hagatna, Guam.
In the 1950s and 1960s we had lists of Communists in the U.S., as that ideology had set itself against us. Today, certain aspects of Islam have set themselves against us, lists of possible adherents once again are useful.
