opinion: Mb. Justice Harlan
  delivered the opinion of the Court.
  William Spinelli was convicted under 18 U. S. C. i 1952 of traveling to St. Louis, Missouri, from a nearby Illinois suburb with the intention of conducting gambling activities proscribed by Missouri law. See Mo. Rev. Stat. § 563.360 (1959). At every appropriate stage in the proceedings in the lower courts, the petitioner challenged the constitutionality of the warrant which authorized the FBI search that uncovered the evidence necessary for his conviction. At each stage, Spinelli’s challenge was treated in a different way. At a pretrial suppression hearing, the United States District Court for the Eastern District of Missouri held that Spinelli lacked standing to raise a Fourth Amendment objection. A unanimous panel of the Court of Appeals for the Eighth Circuit rejected the District Court’s ground, a majority holding further that the warrant was issued without probable cause. After an en banc rehearing, the Court of Appeals sustained the warrant and affirmed the conviction by a vote of six to two. 382 F. 2d 871. Both the majority and dissenting en banc opinions reflect a most conscientious effort to apply the principles we announced in Aguilar v. Texas, 378 U. S. 108 (1964), to a factual situation whose basic characteristics have not been at all uncommon in recent search warrant cases. Believing it desirable that the principles of Aguilar should be further explicated, we granted certiorari, 390 U. S. 942, our writ being later limited to the question of the constitutional validity of the search and seizure. 391 U. S. 933. For reasons that follow we reverse.
  In Aguilar, a search warrant had issued upon an affidavit of police officers who swore only that they had “received reliable information from a credible person and do believe” that narcotics were being illegally stored on the described premises. While recognizing that the constitutional requirement of probable cause can be satisfied by hearsay information, this Court held the affidavit inadequate for two reasons. First, the application failed to set forth any of the “underlying circumstances” necessary to enable the magistrate independently to judge of the validity of the informant’s conclusion that the narcotics were where he said they were. Second, the affiant-officers did not attempt to support their claim that their informant was “ 'credible’ or his information 'reliable.’ ” The Government is, however, quite right in saying that the FBI affidavit in the present case is more ample than that in Aguilar. Not only does it contain a report from an anonymous informant, but it also contains a report of an independent FBI investigation which is said to corroborate the informant’s tip. We are, then, required to delineate the manner in which Aguilar’s two-pronged test should be applied in these circumstances.
  In essence, the affidavit, reproduced in full in the Appendix to this opinion, contained the following allegations:
  1. The FBI had kept track of Spinelli’s movements on five days during the month of August 1965. On four of these occasions, Spinelli was seen crossing one of two bridges leading from Illinois into St. Louis, Missouri, between 11 a. m. and 12:15 p. m. On four of the five days, Spinelli was also seen parking his car in a lot used by residents of an apartment house at 1108 Indian Circle Drive in St. Louis, between 3:30 p. m. and 4:45 p. m.
  On one day, Spinelli was followed further and seen to enter a particular apartment in the building.
  2. An FBI check with the telephone company revealed that this apartment contained two telephones listed under the name of Grace P. Hagen, and carrying the numbers WYdown 4-0029 and WYdown 4-0136.
  3. The application stated that “William Spinelli is known to this affiant and to federal law enforcement agents and local law enforcement agents as a bookmaker, an associate of bookmakers, a gambler, and an associate of gamblers.”
  4. Finally, it was stated that the FBI “has been informed by a confidential reliable informant that William Spinelli is operating a handbook and accepting wagers and disseminating wagering information by means of the telephones which have been assigned the numbers WYdown 4-0029 and WYdown 4-0136.”
  There can be no question that the last item mentioned, detailing the informant’s tip, has a fundamental place in this warrant application. Without it, probable cause could not be established. The first two items reflect only innocent-seeming activity and data. Spinelli’s travels to and from the apartment building and his entry into a particular apartment on one occasion could hardly be taken as bespeaking gambling activity; and there is surely nothing unusual about an apartment containing two separate telephones. Many a householder indulges himself in this petty luxury. Finally, the allegation that Spinelli was “known” to the affiant and to other federal and local law enforcement officers as a gambler and an associate of gamblers is but a bald and unilluminating assertion of suspicion that is entitled to no weight in appraising the magistrate’s decision. Nathanson v. United States, 290 U. S. 41, 46 (1933).
  So much indeed the Government does not deny. Rather, following the reasoning of the Court of Appeals, the Government claims that the informant’s tip gives a suspicious color to the FBI’s reports detailing Spinelli’s innocent-seeming conduct and that, conversely, the FBI’s surveillance corroborates the informant’s tip, thereby entitling it to more weight. It is true, of course, that the magistrate is obligated to render a judgment based upon a common-sense reading of the entire affidavit. United States v. Ventresca, 380 U. S. 102, 108 (1965). We believe, however, that the “totality of circumstances” approach taken by the Court of Appeals paints with too broad a brush. Where, as here, the informer’s tip is a necessary element in a finding of probable cause, its proper weight must be determined by a more precise analysis.
  The informer’s report must first be measured against Aguilar’s standards so that its probative value can be assessed. If the tip is found inadequate under Aguilar, the other allegations which corroborate the information contained in the hearsay report should then be considered. At this stage as well, however, the standards enunciated in Aguilar must inform the magistrate’s decision. He must ask: Can it fairly be said that the tip, even when certain parts of it have been corroborated by independent sources, is as trustworthy as a tip which would pass Aguilar’s tests without independent corroboration? Aguilar is relevant at this stage of the inquiry as well because the tests it establishes were designed to implement the long-standing principle that probable cause must be determined by a “neutral and detached magistrate,” and not by “the officer engaged in the often competitive enterprise of ferreting out crime.” Johnson v. United States, 333 U. S. 10, 14 (1948). A magistrate cannot be said to have properly discharged his constitutional duty if he relies on an informer’s tip which — even when partially corroborated — is not as reliable as one which passes Aguilar’s requirements when standing alone.
  Applying these principles to the present case, we first consider the weight to be given the informer’s tip when it is considered apart from the rest of the affidavit. It is clear that a Commissioner could not credit it without abdicating his constitutional function. Though the affi-ant swore that his confidant was “reliable,” he offered the magistrate no reason in support of this conclusion. Perhaps even more important is the fact that Aguilar’s other test has not been satisfied. The tip does not contain a sufficient statement of the underlying circumstances from which the informer concluded that Spinelli was running a bookmaking operation. We are not told how the FBI’s source received his information — it is not alleged that the informant personally observed Spinelli at work or that he had ever placed a bet with him. Moreover, if the informant came by the information indirectly, he did not explain why his sources were reliable. Cf. Jaben v. United States, 381 U. S. 214 (1965). In the absence of a statement detailing the manner in which the information was gathered, it is especially important that the tip describe the accused’s criminal activity in sufficient detail that the magistrate may know that he is relying on something more substantial than a casual rumor circulating in the underworld or an accusation based merely on an individual’s general reputation.
  The detail provided by the informant in Draper v. United States, 358 U. S. 307 (1959), provides a suitable benchmark. While Hereford, the Government’s informer in that case, did not state the way in which he had obtained his information, he reported that Draper had gone to Chicago the day before by train and that he would return to Denver by train with three ounces of heroin on one of two specified mornings. Moreover, Hereford went on to describe, with minute particularity, the clothes that Draper would be wearing upon his arrival at the Denver station. A magistrate, when confronted with such detail, could reasonably infer that the informant had gained his information in a reliable way. Such an inference cannot be made in the present case. Here, the only facts supplied were that Spinelli was using two specified telephones and that these phones were being used in gambling operations. This meager report could easily have been obtained from an offhand remark heard at a neighborhood bar.
  Nor do we believe that the patent doubts Aguilar raises as to the report’s reliability are adequately resolved by a consideration of the allegations detailing the FBI’s independent investigative efforts. At most, these allegations indicated that Spinelli could have used the telephones specified by the informant for some purpose. This cannot by itself be said to support both the inference that the informer was generally trustworthy and that he had made his charge against Spinelli on the basis of information obtained in a reliable way. Once again, Draper provides a relevant comparison. Independent police work in that case corroborated much more than one small detail that had been provided by the informant. There, the police, upon meeting the inbound Denver train on the second morning specified by informer Hereford, saw a man whose dress corresponded precisely to Hereford’s detailed description. It was then apparent that the informant had not been fabricating his report out of whole cloth; since the report was of the sort which in common experience may be recognized as having been obtained in a reliable way, it was perfectly clear that probable cause had been established.
  We conclude, then, that in the present case the informant’s tip — even when corroborated to the extent indicated — was not sufficient to provide the basis for a finding of probable cause. This is not to say that the tip was so insubstantial that it could not properly have counted in the magistrate’s determination. Rather, it needed some further support. When we look to the other parts of the application, however, we find nothing alleged which would permit the suspicions engendered by the informant’s report to ripen into a judgment that a crime was probably being committed. As we have already seen, the allegations detailing the FBI’s surveillance of Spinelli and its investigation of the telephone company records contain no suggestion of criminal conduct when taken by themselves — and they are not endowed with an aura of suspicion by virtue of the informer’s tip. Nor do we find that the FBI’s reports take on a sinister color when read in light of common knowledge that bookmaking is often carried on over the telephone and from premises ostensibly used by others for perfectly normal purposes. Such an argument would carry weight in a situation in which the premises contain an unusual number of telephones or abnormal activity is observed, cf. McCray v. Illinois, 386 U. S. 300, 302 (1967), but it does not fit this case where neither of these factors is present. All that remains to be considered is the flat statement that Spinelli was “known” to the FBI and others as a gambler. But just as a simple assertion of police suspicion is not itself a sufficient basis for a magistrate’s finding of probable cause, we do not believe it may be used to give additional weight to allegations that would otherwise be insufficient.
  The affidavit, then, falls short of the standards set forth in Aguilar, Draper, and our other decisions that give content to the notion of probable cause. In holding as we have done, we do not retreat from the established propositions that only the probability, and not a prima facie showing, of criminal activity is the standard of probable cause, Beck v. Ohio, 379 U. S. 89, 96 (1964); that affidavits of probable cause are tested by much less rigorous standards than those governing the admissibility of evidence at trial, McCray v. Illinois, 386 U. S. 300, 311 (1967); that in judging probable cause issuing magistrates are not to be confined by niggardly limitations or by restrictions on the use of their common sense, United States v. Ventresca, 380 U. S. 102, 108 (1965); and that their determination of probable cause should be paid great deference by reviewing courts, Jones v. United States, 362 U. S. 257, 270-271 (1960). But we cannot sustain this warrant without diluting important safeguards that assure that the judgment of a disinterested judicial officer will interpose itself between the police and the citizenry.
  The judgment of the Court of Appeals is reversed and the case is remanded to that court for further proceedings consistent with this opinion.
  It is so ordered.
  Me. Justice Marshall took no part in the consideration or decision of this case.
  APPENDIX TO OPINION OF THE COURT.
  Affidavit in Support of Search Warrant.
  I, Robert L. Bender, being duly sworn, depose and say that I am a Special Agent of the Federal Bureau of Investigation, and as such am authorized to make searches and seizures.
  That on August 6, 1965, at approximately 11:44 a. m., William Spinelli was observed by an Agent of the Federal Bureau of Investigation driving a 1964 Ford convertible, Missouri license HC3-649, onto the Eastern approach of the Veterans Bridge leading from East St. Louis, Illinois, to St. Louis, Missouri.
  That on August 11, 1965, at approximately 11:16 a. m., William Spinelli was observed by an Agent of the Federal Bureau of Investigation driving a 1964 Ford convertible, Missouri license HC3-649, onto the Eastern approach of the Eads Bridge leading from East St. Louis, Illinois, to St. Louis, Missouri.
  Further, at approximately 11:18 a. m. on August 11, 1965, I observed William Spinelli driving the aforesaid Ford convertible from the Western approach of the Eads Bridge into St. Louis, Missouri.
  Further, at approximately 4:40 p. m. on August 11, 1965, I observed the aforesaid Ford convertible, bearing Missouri license HC3-649, parked in a parking lot used by residents of The Chieftain Manor Apartments, approximately one block east of 1108 Indian Circle Drive.
  On August 12, 1965, at approximately 12:07 p. m., William. Spinelli was observed by an Agent of the Federal Bureau of Investigation driving the aforesaid 1964 Ford convertible onto the Eastern approach of the Veterans Bridge from East St. Louis, Illinois, in the direction of St. Louis, Missouri.
  Further, on August 12, 1965, at approximately 3:46 p. m., I observed William Spinelli driving the aforesaid 1964 Ford convertible onto the parking lot used by the residents of The Chieftain Manor Apartments approximately one block east of 1108 Indian Circle Drive.
  Further, on August 12, 1965, at approximately 3:49 p. m., William Spinelli was observed by an Agent of the Federal Bureau of Investigation entering the front entrance of the two-story apartment building located at 1108 Indian Circle Drive, this building being one of The Chieftain Manor Apartments.
  On August 13, 1965, at approximately 11:08 a. m., William Spinelli was observed by an Agent of the Federal Bureau of Investigation driving the aforesaid Ford convertible onto the Eastern approach of the Eads Bridge from East St. Louis, Illinois, heading towards St. Louis, Missouri.
  Further, on August 13, 1965, at approximately 11:11 a. m., I observed William Spinelli driving the aforesaid Ford convertible from the Western approach of the Eads Bridge into St. Louis, Missouri.
  Further, on August 13, 1965, at approximately 3:45 p. m., I observed William Spijielli driving the aforesaid 1964 Ford convertible onto the parking area used by residents of The Chieftain Manor Apartments, said parking area being approximately one block from 1108 Indian Circle Drive.
  Further, on August 13, 1965, at approximately 3:55 p. m., William Spinelli was observed by an Agent of the Federal Bureau of Investigation entering the corner apartment located on the second floor in the southwest corner, known as Apartment F, of the two-story apartment building known and numbered as 1108 Indian Circle Drive.
  On August 16, 1965, at approximately 3:22 p. m., I observed William Spinelli driving the aforesaid Ford convertible onto the parking lot used by the residents of The Chieftain Manor Apartments approximately one block east of 1108 Indian Circle Drive.
  Further, an Agent of the F. B. I. observed William Spinelli alight from the aforesaid Ford convertible and walk toward the apartment building located at 1108 Indian Circle Drive.
  The records of the Southwestern Bell Telephone Company reflect that there are two telephones located in the southwest corner apartment on the second floor of the apartment building located at 1108 Indian Circle Drive under the name of Grace P. Hagen. The numbers listed in the Southwestern Bell Telephone Company records for the aforesaid telephones are WYdown 4-0029 and WYdown 4-0136.
  William Spinelli is known to this affiant and to federal law enforcement agents and local law enforcement agents as a bookmaker, an associate of bookmakers, a gambler, and an associate of gamblers.
  The Federal Bureau of Investigation has been informed by a confidential reliable informant that William Spinelli is operating a handbook and accepting wagers and disseminating wagering information by means of the telephones which have been assigned the numbers WYdown 4-0029 and WYdown 4-0136.
  /s/Robert L. Bender,
  Robert L. Bender,
  Special Agent, Federal Bureau of Investigation.
  Subscribed and sworn to before me this 18th day of August, 1965, at St. Louis, Missouri.
  /s/ William R. O’Toole.
  The relevant portion of the statute reads:
  “(a) Whoever travels in interstate or foreign commerce or uses any facility in interstate . . . commerce . . . with intent to—
  “(3) otherwise promote, manage, establish, carry on . . . any unlawful activity, and thereafter performs or attempts to perform any of the acts specified in subparagraphs (1), (2), and (3), shall be fined not more than $10,000 or imprisoned for not more than five years, or both.
  “(b) As used in this section ‘unlawful activity’ means (1) any business enterprise involving gambling ... in violation of the laws of the State in which they are committed or of the United States . .. .”
  We agree with the Court of Appeals that Spinelli has standing to raise his Fourth Amendment claim. The issue arises because at the time the FBI searched the apartment in which Spinelli was alleged to be conducting his bookmaking operation, the petitioner was not on the premises. Instead, the agents did not execute their search warrant until Spinelli was seen to leave the apartment, lock the door, and enter the hallway. At that point, petitioner was arrested, the key to the apartment was demanded of him, and the search commenced. Since petitioner would plainly have standing if he had been arrested inside the apartment, Jones v. United States, 362 U. S. 257, 267 (1960), it cannot matter that the agents preferred to delay the arrest until petitioner stepped into the hallway — especially when the FBI only managed to gain entry into the apartment by requiring petitioner to surrender his key.
  It is, of course, of no consequence that the agents might have had additional information which could have been given to the Commissioner. “It is elementary that in passing on the validity of a warrant, the reviewing court may consider only information brought to the magistrate’s attention.” Aguilar v. Texas, 378 U. S. 108, 109, n. 1 (emphasis in original). Since the Government does not argue that whatever additional information the agents may have possessed was sufficient to provide probable cause for the arrest, thereby justifying the resultant search as well, we need not consider that question.
  No report was made as to Spinelli’s movements during the period between his arrival in St. Louis at noon and his arrival at the parking lot in the late afternoon. In fact, the evidence at trial indicated that Spinelli frequented the offices of his stockbroker during this period.
  While Draper involved the question whether the police had probable cause for an arrest without a warrant, the analysis required for an answer to this question is basically similar to that demanded of a magistrate when he considers whether a search warrant should issue.
  A box containing three uninstalled telephones was found in the apartment, but only after execution of the search warrant.
  In those cases in which this Court has found probable cause established, the showing made was much more substantial than the one made here. Thus, in United States v. Ventresca, 380 U. S. 102, 104 (1965), FBI agents observed repeated deliveries of loads of sugar in 60-pound bags, smelled the odor of fermenting mash, and heard “ ‘sounds similar to that of a motor or a pump coming from the direction of Ventresca’s house.” Again, in McCray v. Illinois, 386 U. S. 300, 303-304 (1967), the informant reported that McCray “ ‘was selling narcotics and had narcotics on his person now in the vicinity of 47th and Calumet.’ ” When the police arrived at the intersection, they observed McCray engaging in various suspicious activities. 386 U. S., at 302.
  In the view we have taken of this case, it becomes unnecessary to decide whether the search warrant was properly executed, or whether it sufficiently described the things that were seized.