Congressional Research Service Washington, D.C. 20540-7000
Memorandum January 30, 2006
TO: Senate Select Committee on Intelligence
Attention: Mike Davidson
FROM: American Law Division
SUBJECT: Probable Cause, Reasonable Suspicion, and Reasonableness Standards in the
Context of the Fourth Amendment and the Foreign Intelligence Surveillance
Act
This is in response to your request for a brief description of the Fourth Amendment’s
probable cause, reasonable suspicion, and reasonableness standards. In over simplified
terms, probable cause “exist[s] where the known facts and circumstances are sufficient to
warrant a man of reasonable prudence in the belief that contraband or evidence of a crime
will be found,” Ornelas v. United States, 517 U.S. 690, 696 (1996); Illinois v. Gates, 462
U.S. 213, 238 (1983).
Under a similar gloss, reasonable suspicion is a standard, more than a hunch but
considerably below preponderance of the evidence, which justifies an officer’s investigative
stop of an individual upon the articulable and particularized belief that criminal activity is
afoot, Ornelas v. United States, 517 U.S. at 695; Illinois v. Gates, 462 U.S. at 235.
And Fourth Amendment reasonableness is that point at which the government’s interest
advanced by a particular search or seizure outweighs the loss of individual privacy or
freedom of movement that attends the government’s action, Illinois v. Lidster, 540 U.S. 419,
427 (2004)(“in judging reasonableness, we look to the gravity of the public concerns served
by the seizure, the degree to which the seizure advances the public interest, and the severity
of the interference with individual liberty”).
Again in summary and to add further complication, the Supreme Court has speculated
that in national security cases the “probable cause” may be less demanding or at least
different than it is in the context of a traditional criminal investigation, United States v.
United States District Court, 407 U.S. 297, 322 (1972)(“the gathering of security intelligence
is often long range and involves the interrelation of various sources and types of information.
. . . Thus, the focus of domestic surveillance may be less precise than that directed against
more conventional types of crime”). FISA permits recourse to this reduced application of
the probable cause standard in spy cases but not in terrorism cases, In re Sealed Case, 310
F.3d 717, 739 (F.I.S.Ct.Rev. 2002)(“Congress allowed this lesser showing for clandestine
intelligence activities – but not, notably, for other activities, including terrorism . . .”). Yet
it is focus and not the standard that is different in FISA cases. The standard is the same; the
