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AHTG's Tripartite Alliance Earth timeline describes a dystopian U.S. where an increasingly dictatorial government militarizes the Federal Bureau of Investigation to crack down on militant black liberation rings, and all civil rights groups in general in the 1960s. Eventually it becomes so powerful that the FBI apparently is able to stage a coup in the Philippines of all places (replacing the CIA as America's number-1 agency for foreign dirty tricks?) and having death squads round up political prisoners in the dead of night.
So how could the FBI or any other federal agency become militarized in the real world? They don't have to do this in a necessarily evil manner, but it would probably be a bad sign. It would take a lot of unrest to have a FBI or ATF acting like paramilitaries.