These Fourth Amendment freedoms cannot properly be guaranteed if domestic
security surveillances may be conducted solely within the discretion of the
Executive Branch. The Fourth Amendment does not contemplate the executive
officers of Government as neutral and disinterested magistrates. Their duty and
responsibility are to enforce the laws, to investigate, and to prosecute. Katz
v. United States, supra, at 359-360 (DOUGLAS, J., concurring). But those charged
with this investigative and prosecutorial duty should not be the sole judges of
when to utilize constitutionally sensitive means in pursuing their tasks. The
historical judgment, which the Fourth Amendment accepts, is that unreviewed
executive discretion may yield too readily to pressures to obtain incriminating
evidence and overlook potential invasions of privacy and protected speech.

It may well be that, in the instant case, the Government's surveillance
of Plamondon's conversations was a reasonable one which readily would have
gained prior judicial approval. But this Court "has never sustained a search
upon the sole ground that officers reasonably expected to find evidence of a
particular crime and voluntarily confined their activities to the least
intrusive means consistent with that end." Katz, supra, at 356-357. The Fourth
Amendment contemplates a prior judicial judgment, not the risk that executive
discretion may be reasonably exercised. This judicial role accords with our
basic constitutional doctrine that individual freedoms will best be preserved
through a separation of powers and division of functions among the different
branches and levels of Government. Harlan, Thoughts at a Dedication: Keeping the
Judicial Function in Balance, 49 A. B. A. J. 943-944 (1963). The independent
check upon executive discretion is not satisfied, as the Government argues,
by "extremely limited" post-surveillance judicial review. Indeed,
post-surveillance review would never reach the surveillances which failed to
result in prosecutions. Prior review by a neutral and detached magistrate is the
time-tested means of effectuating Fourth Amendment rights. Beck v. Ohio, 379
U.S. 89, 96 (1964).

It is true that there have been some exceptions to the warrant
requirement. But those exceptions are few in number and carefully
delineated, Katz, supra, at 357; in general, they serve the legitimate needs of
law enforcement officers to protect their own well-being and preserve evidence
from destruction. Even while carving out those exceptions, the Court has
reaffirmed the principle that the "police must, whenever practicable, obtain
advance judicial approval of searches and seizures through the warrant
procedure," Terry v. Ohio, supra, at 20; Chimel v. California, supra, at 762.

The Government argues that the special circumstances applicable to domestic
security surveillances necessitate a further exception to the warrant
requirement. It is urged that the requirement of prior judicial review would
obstruct the President in the discharge of his constitutional duty to protect
domestic security. We are told further that these surveillances are directed
primarily to the collecting and maintaining of intelligence with respect to
subversive forces, and are not an attempt to gather evidence for specific
criminal prosecutions. It is said that this type of surveillance should not be
subject to traditional warrant requirements which were established to govern
investigation of criminal activity, not ongoing intelligence gathering. Brief
for United States 15-16, 23-24; Reply Brief for United States 2-3.

The Government further insists that courts "as a practical matter
would have neither the knowledge nor the techniques necessary to determine
whether there was probable cause to believe that surveillance was necessary to
protect national security." These security problems, the Government contends,
involve "a large number of complex and subtle factors" beyond the competence of
courts to evaluate. Reply Brief for United States 4.

As a final reason for exemption from a warrant requirement, the Government
believes that disclosure to a magistrate of all or even a significant portion of
the information involved in domestic security surveillances "would create
serious potential dangers to the national security and to the lives of
informants and agents. . . . Secrecy is the essential ingredient in intelligence
gathering; requiring prior judicial authorization would create a greater `danger
of leaks . . ., because in addition to the judge, you have the clerk, the
stenographer and some other officer like a law assistant or bailiff who may be
apprised of the nature' of the surveillance." Brief for United States 24-25.

These contentions in behalf of a complete exemption from the warrant
requirement, when urged on behalf of the President and the national security in
its domestic implications, merit the most careful consideration. We certainly do
not reject them lightly, especially at a time of worldwide ferment and when
civil disorders in this country are more prevalent than in the less
turbulent periods of our history. There is, no doubt, pragmatic force to
the Government's position.

But we do not think a case has been made for the requested departure
from Fourth Amendment standards. The circumstances described do not justify
complete exemption of domestic security surveillance from prior judicial
scrutiny. Official surveillance, whether its purpose be criminal investigation
or ongoing intelligence gathering, risks infringement of constitutionally
protected privacy of speech. Security surveillances are especially sensitive
because of the inherent vagueness of the domestic security concept, the
necessarily broad and continuing nature of intelligence gathering, and the
temptation to utilize such surveillances to oversee political dissent. We
recognize, as we have before, the constitutional basis of the President's
domestic security role, but we think it must be exercised in a manner compatible
with the Fourth Amendment. In this case we hold that this requires an
appropriate prior warrant procedure.

We cannot accept the Government's argument that internal security matters
are too subtle and complex for judicial evaluation. Courts regularly deal with
the most difficult issues of our society. There is no reason to believe that
federal judges will be insensitive to or uncomprehending of the issues involved
in domestic security cases. Certainly courts can recognize that domestic
security surveillance involves different considerations from the surveillance of
"ordinary crime." If the threat is too subtle or complex for our senior law
enforcement officers to convey its significance to a court, one may question
whether there is probable cause for surveillance.

Nor do we believe prior judicial approval will fracture the secrecy
essential to official intelligence gathering. The investigation of criminal
activity has long involved imparting sensitive information to judicial
officers who have respected the confidentialities involved. Judges may be
counted upon to be especially conscious of security requirements in national
security cases....

Thus, we conclude that the Government's concerns do not justify departure
in this case from the customary Fourth Amendment requirement of judicial
approval prior to initiation of a search or surveillance. Although some added
burden will be imposed upon the Attorney General, this inconvenience is
justified in a free society to protect constitutional values. Nor do we think
the Government's domestic surveillance powers will be impaired to any
significant degree. A prior warrant establishes presumptive validity of the
surveillance and will minimize the burden of justification in post-surveillance
judicial review. By no means of least importance will be the reassurance of the
public generally that indiscriminate wiretapping and bugging of law-abiding
citizens cannot occur.