So if the FBI can arrest them, they're criminals and we should follow criminal procedure, but if the FBI says they can't arrest them, then we "take action" (what action)?
The original author said: "I don't want the NSA to perform domestic civilian law enforcement functions"
In other words, the FBI (and police, etc) perform domestic law enforcement, following criminal procedure. The NSA, CIA, etc handle non-domestic (foreign) threats, in whatever manner they see fit.
