opinion: McDONALD et al. v. UNITED STATES.
  No. 36.
  Argued October 13, 1948.
  Decided December 13, 1948.
  Charles E. Ford and John Lewis Smith, Jr. argued the cause and filed a brief for petitioners.
  Frederick Bernays Wiener argued the cause for the United States. With him on the brief were Solicitor General Perlman, Robert S. Erdahl and Beatrice Rosenberg.
  Mr. Justice Douglas
  delivered the opinion of the Court.
  Petitioners were convicted in the District Court on evidence obtained by a search made without a warrant. The Court of Appeals affirmed on a divided vote. 83 U. S. App. D. C. 96, 166 F. 2d 957. We brought the case here on certiorari because of doubts whether that result squared with Johnson v. United States, 333 U. S. 10, and Trupiano v. United States, 334 U. S. 699.
  Petitioners were tried without a jury in the District Court for the District of Columbia on an indictment in four counts, charging offenses of carrying on a lottery known as the numbers game in violation of 22 D. C. Code §§ 1501, 1502, 1504 (1940). They were found guilty on all counts.
  Petitioner McDonald, who had previously been arrested for numbers operations, had been under police observation for several months prior to the arrest. During this period and while he was maintaining a home in the District of Columbia, he rented a room in the residence of a Mrs. Terry, who maintained a rooming house in the District. His comings and goings at this address were under surveillance by the police for about two months. They had observed him enter the rooming house during the hours in which operations at the headquarters of the numbers game are customarily carried on.
  On the day of the arrest three police officers surrounded the house. This was midafternoon. They did not have a warrant for arrest nor a search warrant. While outside the house, one of the officers thought that he heard an adding machine. These machines are frequently used in the numbers operation. Believing that the numbers game was in process, the officers sought admission to the house.
  One of them opened a window leading into the landlady’s room and climbed through. He identified himself to her and admitted the other officers to the house.
  After searching the rooms on the ground floor, they proceeded to the second floor. The door of an end bedroom was closed. But one of the officers stood on a chair and looked through the transom. He observed both petitioners in the room, as well as numbers slips, money piled on the table, and adding machines. He yelled to McDonald to open the door and McDonald did so. Both petitioners were arrested, and the officers seized the machines, a suitcase of papers, and money. Whether these machines and papers should have been suppressed as evidence and returned to petitioner McDonald is the major question presented.
  The Fourth Amendment to the Constitution provides: “The right of the people to be secure in their persons, houses, papers, and effects, against unreasonable searches and seizures, shall not be violated, and no Warrants shall issue, but upon probable cause, supported by Oath or affirmation, and particularly describing the place to be searched, and the persons or things to be seized.” This guarantee of protection against unreasonable searches and seizures extends to the innocent and guilty alike. It marks the right of privacy as one of the unique values of our civilization and, with few exceptions, stays the hands of the police unless they have a search warrant issued by a magistrate on probable cause supported by oath or affirmation. And the law provides as a sanction against the flouting of this constitutional safeguard the suppression of evidence secured as a result of the violation, when it is tendered in a federal court. Weeks v. United States, 232 U. S. 383.
  The prosecution seeks to build the lawfulness of the search on the lawfulness of the arrest and so justify the search and seizure without a warrant. See Agnello v. United States, 269 U. S. 20, 30; Harris v. United States, 331 U. S. 145, 150-151. The reasoning runs as follows: Although it was an invasion of privacy for the officers to enter Mrs. Terry's room, that was a trespass which violated her rights under the Fourth Amendment, not McDonald's. Therefore so far as he was concerned, the officers were lawfully within the hallway, as much so as if Mrs. Terry had admitted them. Looking over the transom was not a search, for the eye cannot commit the trespass condemned by the Fourth Amendment. Since the officers observed McDonald in the act of committing an offense, they were under a duty then and there to arrest him. See 4 D. C. Code §§ 140, 143 (1940). The arrest being valid the search incident thereto was lawful.
  We do not stop to examine that syllogism for flaws. Assuming its correctness, we reject the result.
  This is not a case where the officers, passing by on the street, hear a shot and a cry for help and demand entrance in the name of the law. They had been following McDonald and keeping him under surveillance for two months at this rooming house. The prosecution now tells us that the police had no probable cause for obtaining a warrant until,. shortly before the arrest, they heard the sound of the adding machine coming from the rooming house. And there is vague and general testimony in the record that on previous occasions the officers had sought search warrants but had been denied them. But those statements alone do not lay the proper foundation for dispensing with a search warrant.
  Where, as here, officers are not responding to an emergency, there must be compelling reasons to justify the absence of a search warrant. A search without a warrant demands exceptional circumstances, as we held in Johnson v. United States, supra. We will not assume that where a defendant has been under surveillance for months, no search warrant could have been obtained. What showing these officers made when they applied on the earlier occasions, the dates of these applications, and all the circumstances bearing upon the necessity to make this search without a warrant are absent from this record. We cannot allow the constitutional barrier that protects the privacy of the individual to be hurdled so easily. Moreover, when we move to the scene of the crime, the reason for the absence of a search warrant is even less obvious. When the officers heard the adding machine and, at the latest, when they saw what was transpiring in the room, they certainly had adequate grounds for seeking a search warrant.
  Here, as in Johnson v. United States and Trupiano v. United States, the defendant was not fleeing or seeking to escape. Officers were there to apprehend petitioners in case they tried to leave. Nor was the property in the process of destruction nor as likely to be destroyed as the opium paraphernalia in the Johnson case. Petitioners were busily engaged in their lottery venture. No reason, except inconvenience of the officers and delay in preparing papers and getting before a magistrate, appears for the failure to seek a search warrant. But those reasons are no justification for by-passing the constitutional requirement, as we held in Johnson v. United States, supra, p. 15.
  We are not dealing with formalities. The presence of a search warrant serves a high function. Absent some grave emergency, the Fourth Amendment has interposed a magistrate between the citizen and the police. This was done not to shield criminals nor to make the home a safe haven for illegal activities. It was done so that an objective mind might weigh the need to invade that privacy in order to enforce the law. The right of privacy was deemed too precious to entrust to the discretion of those whose job is the detection of crime and the arrest of criminals. Power is a heady thing; and history shows that the police acting on their own cannot be trusted. And so the Constitution requires a magistrate to pass on the desires of the police before they violate the privacy of the home. We cannot be true to that constitutional requirement and excuse the absence of a search warrant ' without a showing by those who seek exemption from the constitutional mandate that the exigencies of the situation made that course imperative.
  It follows from what we have said that McDonald’s motion for suppression of the evidence and the return of the property to him should have been granted. Weeks v. United States, supra; Go-Bart Importing Co. v. United States, 282 U. S. 344, 358. It was, however, denied and the unlawfully seized evidence was used not only against McDonald but against Washington as well, the two being tried jointly. Apart from this evidence there seems to have been little or none against Washington. Even though we assume, without deciding, that Washington, who was a guest of McDonald, had no right of privacy that was broken when the officers searched McDonald’s room without a warrant, we think that the denial of McDonald’s motion was error that was prejudicial to Washington as well. In this case, unlike Agnello v. United States, supra, p. 35, the unlawfully seized materials were the basis of evidence used against the codefendant. If the property had been returned to McDonald, it would not have been available for use at the trial. We can only speculate as to whether other evidence which might have been used against Washington would have been equally probative.
  Reversed.
  Mr. Justice Black concurs in the result.
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