opinion: Mr. Justice Harlan
  delivered the opinion of the Court.
  Petitioner was convicted of the unlawful purchase of narcotics, see 26 U. S. C. (Supp. V) § 4704, after a trial without a jury before the Federal District Court for the Southern District of Texas. A divided Court of Appeals affirmed. 241 F. 2d 575. We granted certiorari to consider petitioner’s challenge to the legality of his arrest and the admissibility in evidence of the narcotics seized from his person at the time of the arrest. 355 U. S. 811.
  Agent Finley of the Federal Bureau of Narcotics obtained a warrant for the arrest of petitioner from the United States Commissioner in Houston, Texas, on January 26, 1956. This warrant, issued under Rules 3 and 4 of the Federal Rules of Criminal Procedure (see note 3, infra), was based on a written complaint, sworn to by Finley, which read in part:
  “The undersigned complainant [Finley] being duly sworn states: That on or about January 26, 1956, at Houston, Texas in the Southern District of Texas, Veto Giordenello did receive, conceal, etc., narcotic drugs, to-wit: heroin hydrochloride with knowledge of unlawful importation; in violation of Section 174, Title 21, United States Code.
  “And the complainant further states that he believes that-are material witnesses in relation to this charge.”
  About 6 o’clock in the afternoon of the following day, January 27, Finley saw petitioner drive up to his residence in a car and enter the house. He emerged shortly thereafter and drove away in the same car, closely followed in a second car by a person described by Finley as a “well-known police character.” Finley pursued the cars until they stopped near another residence which was entered by petitioner. When petitioner left this residence, carrying a brown paper bag in his hand, and proceeded towards his car, Finley executed the arrest warrant and seized the bag, which proved to contain a mixture of heroin and other substances. Although warned of his privilege to remain silent, petitioner promptly admitted purchasing the heroin in Chicago and transporting it to Houston.
  On January 28 petitioner appeared with counsel before a United States Commissioner. He waived the preliminary examination contemplated by Rule 5 of the Rules of Criminal Procedure, see p. 483, infra, and was arraigned on the complaint upon which the arrest warrant had been issued on January 26. Prior to trial petitioner, alleging for the first time that his arrest and the coincident seizure from his person of the paper bag were illegal, moved to suppress for use as evidence the heroin found in the bag. This motion was denied by the District Court, and petitioner’s conviction and its affirmance by the Court of Appeals followed.
  In this Court petitioner argues, as he did below, that Finley’s seizure of the heroin was unlawful, since the warrant of arrest was illegal and the seizure could be justified only as incident to a legal arrest, and that consequently the admission of the heroin into evidence was error which requires that his conviction be set aside. The Government contends that petitioner waived his right to challenge the legality of his arrest, and hence to object to the admissibility of this evidence, by failing to question the sufficiency of the warrant at the time he was brought before the United States Commissioner. It further asserts that the arrest warrant satisfied the Federal Rules of Criminal Procedure, and, alternatively, that the arrest can be sustained apart from the warrant because Finley had probable cause to believe that petitioner had committed a felony. The Government recognizes that since Finley had no search warrant, the heroin was admissible in evidence only if its seizure was incident to a lawful arrest, see United States v. Rabinowitz, 339 U. S. 56, 60, and that if the arrest was illegal the admission of this evidence was reversible error.
  I.
  We think it clear that petitioner, by waiving preliminary examination before the United States Commissioner, did not surrender his right subsequently to contest in court the validity of the warrant on the grounds here asserted. A claim of this nature may involve legal issues of subtlety and complexity which it would be unfair to require a defendant to present so soon after arrest, and in many instances, as here, before his final selection of counsel.
  In addition, examination of the purpose of the preliminary examination before a Commissioner makes evident the unsoundness of the Government’s position. Rule 5 (c) of the Federal Rules of Criminal Procedure provides in part:
  “If from the evidence it appears to the commissioner that there is probable cause to believe that an offense has been committed and that the defendant has committed it, the commissioner shall forthwith hold him to answer in the district court; otherwise the commissioner shall discharge him.”
  By waiving preliminary examination, a defendant waives no more than the right which this examination was intended to secure him — the right not to be held in the absence of a finding by the Commissioner of probable cause that he has committed an offense.
  By the same token, the Commissioner here had no authority to adjudicate the admissibility at petitioner’s later trial of the heroin taken from his person. That issue was for the trial court. This is specifically recognized by Rule 41 (e) of the Criminal Rules, which provides that a defendant aggrieved by an unlawful search and seizure may “. . . move the district court ... to suppress for use as evidence anything so obtained on the ground that . . .” the arrest warrant was defective on any of several grounds. This was the procedural path followed by petitioner, and we hold it proper to put in issue the legality of the warrant. Cf. Albrecht v. United States, 273 U. S. 1, 9-11.
  II.
  Petitioner challenges the sufficiency of the warrant on two grounds: (1) that the complaint on which the warrant was issued was inadequate because the complaining officer, Finley, relied exclusively upon hearsay information rather than personal knowledge in executing the complaint; and (2) that the complaint was in any event defective in that it in effect recited no more than the elements of the crime charged, namely the concealment of heroin with knowledge of its illegal importation in violation of 21 U. S. C. § 174.
  It appears from Finley’s testimony at the hearing on the suppression motion that until the warrant was issued on January 26 his suspicions of petitioner’s guilt derived entirely from information given him by law enforcement officers and other persons in Houston, none of whom either appeared before the Commissioner or submitted affidavits. But we need not decide whether a warrant may be issued solely on hearsay information, for in any event we find this complaint defective in not providing a sufficient basis upon which a finding of probable cause could be made.
  Criminal Rules 3 and 4 provide that an arrest warrant shall be issued only upon a written and sworn eomplaint (1) setting forth "the essential facts constituting the offense charged,” and (2) showing “that there is probable cause to believe that [such] an offense has been committed and that the defendant has committed it . ...” The provisions of these Rules must be read in light of the constitutional requirements they implement. The language of the Fourth Amendment, that . . no Warrants shall issue, but upon probable cause, supported by Oath or affirmation, and particularly describing . . . the persons or things to be seized,” of course applies to arrest as well as search warrants. See Ex parte Burford, 3 Cranch 448; McGrain v. Daugherty, 273 U. S. 135, 154-157. The protection afforded by these Rules, when they are viewed against their constitutional background, is that the inferences from the facts which lead to the complaint “. . . be drawn by a neutral and detached magistrate instead of being judged by the officer engaged in the often competitive enterprise of ferreting out crime.” Johnson v. United States, 333 U. S. 10, 14. The purpose of the complaint, then, is to enable the appropriate magistrate, here a Commissioner, to determine whether the “probable cause” required to support a warrant exists. The Commissioner must judge for himself the persuasiveness of the facts relied on by a complaining officer to show probable cause. He should not accept without question the complainant’s mere conclusion that the person whose arrest is sought has committed a crime.
  When the complaint in this case is judged with these considerations in mind, it is clear that it does not pass muster because it does not provide any basis for the Commissioner’s determination under Rule 4 that probable cause existed. The complaint contains no affirmative allegation that the affiant spoke with personal knowledge of the matters contained therein; it does not indicate any sources for the complainant’s belief; and it does not set forth any other sufficient basis upon which a finding of probable cause could be made. We think these deficiencies could not be cured by the Commissioner’s reliance upon a presumption that the complaint was made on the personal knowledge of the complaining officer. The insubstantiality of such an argument is illustrated by the facts of this very case, for Finley’s testimony at the suppression hearing clearly showed that he had no personal knowledge of the matters on which his charge was based. In these circumstances, it is difficult to understand how the Commissioner could be expected to assess independently the probability that petitioner committed the crime charged. Indeed, if this complaint were upheld, the substantive requirements would be completely read out of Rule 4, and the complaint would be of only formal significance, entitled to perfunctory approval by the Commissioner. This would not comport with the protective purposes which a complaint is designed to achieve.
  It does not avail the Government to argue that because a warrant of arrest may be issued as of course upon an indictment, this complaint was adequate since its allegations would suffice for an indictment under Federal Rule of Criminal Procedure 7 (c). A warrant of arrest can be based upon an indictment because the grand jury’s determination that probable cause existed for the indictment also establishes that element for the purpose of issuing a warrant for the apprehension of the person so charged. Here, in the absence of an indictment, the issue of probable cause had to be determined by the Commissioner, and an adequate basis for such a finding had to appear on the face of the complaint.
  III.
  In the two lower courts the Government defended the legality of petitioner’s arrest by relying entirely on the validity of the warrant. In this Court, however, its principal contention has been that the arrest was justified apart from the warrant. The argument is that Texas law permits arrest without a warrant upon probable cause that the person arrested has committed a felony; that in the absence of a controlling federal statute, as in the case here, federal officers turn to the law of the State where an arrest is made as the source of their authority to arrest without a warrant, cf. United States v. Di Re, 332 U. S. 581, 589; Johnson v. United States, supra, at 15; and that Finley, on the basis of the facts he testified to before the District Court, must be deemed, within the standards of Texas law, to have had the probable cause necessary to arrest petitioner without a warrant.
  We do not think that these belated contentions are open to the Government in this Court and accordingly we have no occasion to consider their soundness. To permit the Government to inject its new theory into the case at this stage would unfairly deprive petitioner of an adequate opportunity to respond. This is so because in the District Court petitioner, being entitled to assume that the warrant constituted the only purported justification for the arrest, had no reason to cross-examine Finley or to adduce evidence of his own to rebut the contentions that the Government makes here for the first time.
  Nor do we think that it would be sound judicial administration to send the case back to the District Court for a special hearing on the issue of probable cause which would determine whether the verdict of guilty and the judgment already entered should be allowed to stand. The facts on which the Government now relies to uphold the arrest were fully known to it at the time of trial, and there are no special circumstances suggesting such an exceptional course. Cf. United States v. Shotwell Mfg. Co., 355 U. S. 233. This is not to say, however, that in the event of a new trial the Government may not seek to justify petitioner’s arrest without relying on the warrant.
  We hold that the seizure in this case was illegal, that the seized narcotics should therefore not have been admitted into evidence, and that petitioner’s conviction accordingly must be set aside. The judgment of the Court of Appeals is
  Reversed.
  The indictment returned against petitioner did not refer to the crime charged in the complaint but was based on two related offenses. One, charging possession of unlawfully imported narcotics, 21 U. S. C. § 174, was dropped by the Government prior to trial. The other, charging unlawful purchase of narcotics, 26 U. S. C. (Supp. V) § 4704, resulted in petitioner’s conviction.
  It appears that in the courts below petitioner relied primarily, if not entirely, on the first of these grounds. That of course does not prevent him from relying here also on the second ground, which raises simply a question of law as to the sufficiency of the complaint. See United Brotherhood of Carpenters v. United States, 330 U. S. 395, 412; Weems v. United States, 217 U. S. 349, 362; Fed. Rules Crim. Proc., Rule 52 (b).
  Rule 3: “The complaint is a written statement of the essential facts constituting the offense charged. It shall be made upon oath before a commissioner or other officer empowered to commit persons charged with offenses against the United States.”
  Rule 4 (a): . If it appears from the complaint that there is probable cause to believe that an offense has been committed, and that the defendant has committed it, a warrant for the arrest of the defendant shall issue to any officer authorized by law to execute it. . . .”
  The Government asked the District Court to take judicial notice that petitioner was arrested on a “complaint and warrant.” In addition, Finley’s testimony and the “return” of the United States Marshal to the warrant leave no room for doubt that in fact the warrant constituted the basis for petitioner’s arrest.
instruction: What follows is an opinion from the Supreme Court of the United States. Your task is to determine the ideological "direction" of the decision ("liberal", "conservative", or "unspecifiable"). Use "unspecifiable" if the issue does not lend itself to a liberal or conservative description (e.g., a boundary dispute between two states, real property, wills and estates), or because no convention exists as to which is the liberal side and which is the conservative side (e.g., the legislative veto). Specification of the ideological direction comports with conventional usage. In the context of issues pertaining to criminal procedure, civil rights, First Amendment, due process, privacy, and attorneys, consider liberal to be pro-person accused or convicted of crime, or denied a jury trial, pro-civil liberties or civil rights claimant, especially those exercising less protected civil rights (e.g., homosexuality), pro-child or juvenile, pro-indigent pro-Indian, pro-affirmative action, pro-neutrality in establishment clause cases, pro-female in abortion, pro-underdog, anti-slavery, incorporation of foreign territories anti-government in the context of due process, except for takings clause cases where a pro-government, anti-owner vote is considered liberal except in criminal forfeiture cases or those where the taking is pro-business violation of due process by exercising jurisdiction over nonresident, pro-attorney or governmental official in non-liability cases, pro-accountability and/or anti-corruption in campaign spending pro-privacy vis-a-vis the 1st Amendment where the privacy invaded is that of mental incompetents, pro-disclosure in Freedom of Information Act issues except for employment and student records. In the context of issues pertaining to unions and economic activity, consider liberal to be pro-union except in union antitrust where liberal = pro-competition, pro-government, anti-business anti-employer, pro-competition, pro-injured person, pro-indigent, pro-small business vis-a-vis large business pro-state/anti-business in state tax cases, pro-debtor, pro-bankrupt, pro-Indian, pro-environmental protection, pro-economic underdog pro-consumer, pro-accountability in governmental corruption, pro-original grantee, purchaser, or occupant in state and territorial land claims anti-union member or employee vis-a-vis union, anti-union in union antitrust, anti-union in union or closed shop, pro-trial in arbitration. In the context of issues pertaining to judicial power, consider liberal to be pro-exercise of judicial power, pro-judicial "activism", pro-judicial review of administrative action. In the context of issues pertaining to federalism, consider liberal to be pro-federal power, pro-executive power in executive/congressional disputes, anti-state. In the context of issues pertaining to federal taxation, consider liberal to be pro-United States and conservative pro-taxpayer. In miscellaneous, consider conservative the incorporation of foreign territories and executive authority vis-a-vis congress or the states or judcial authority vis-a-vis state or federal legislative authority, and consider liberal legislative veto. In interstate relations and private law issues, consider unspecifiable in all cases.