The answer to this problem was the Foreign Intelligence Surveillance Act (FISA), enacted in 1978. FISA created a "wall" between law enforcement measures aimed at criminals and foreign intelligence actions aimed at agents of a foreign power. For law enforcement, a wiretap required probable cause, and the existence of the wiretap was disclosed to the target after the fact. In addition, overreaching in a wiretap could prevent the information from being used later in a criminal trial.
Now a FISA wiretap is permitted if a "significant" purpose is foreign intelligence, even if there is a large domestic law enforcement reason for surveillance. The standards for getting a FISA wiretap were softened, more intelligence-sharing between the FBI and the CIA was encouraged, and "roving" wiretaps were authorized to track suspects who are using multiple phones or computers to communicate.
One of the alarming aspects of the FISA story is that the FBI's then-secret pattern of misbehavior had so outraged the judges by summer 2001 that prosecutors were reluctant to ask for a FISA warrant to search the computer of suspected hijacker Zacarias Moussaoui. The court's opinion shows one bright line that we should retain. The court permits sharing of FISA data in some instances, but it orders that "law enforcement officials do not direct or control the use of the FISA procedures to enhance criminal prosecution." This decision matches both the Constitution and common sense. When a prosecutor directs someone to do a search on U.S. soil, the Fourth Amendment applies and the usual warrant requirements must apply as well.
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