delivered the opinion of the Court.

Petitioner stands convicted of unlawfully possessing three cartons of radios valued at more than $100 which had been stolen from an interstate shipment. See 18 U. S. C. § 659. The issue in the ease is whether there was probable cause for the arrest leading to the search that produced the evidence on which the conviction rests. A timely motion to suppress the evidence was made by[*99] petitioner and overruled by the District Court; and the judgment of.conviction was affirmed by the Court of Appeals on a divided vote. 259 F. 2d 725. The case is here on a petition for a writ of certiorari, 359 U. S. 904.

There was. a theft from an interstate shipment, of whisky at a terminal in Chicago. The next day two FBI agents were in the neighborhood investigating it. They saw petitioner and one Pierotti walk across a street from a tavern and get into an automobile. The agents had been given, by the employer of Pierotti, information of an undisclosed nature “concerning the implication of the defendant Pierotti with interstate shipments.” But, so far as the record shows, he never went so far as to tell the agents he suspected Pierotti of any such thefts. The agents followed the car and-saw it enter an alley and stop. Petitioner got but of the car, entered a gangway leading to residential premises and returned in a few minutes with some cartons.. He placed them in the car and he and Pierotti drove off. . The agents were unable to follow the car. But later they found it parked at the same place near the tavern. Shortly they saw petitioner and Pierotti leave the tavern, get into the car, and drive off.- The car stopped in the same alley as before; petitioner entered the same gangway and returned with more cartons. The agents observed this transaction-from a distance of some 300 feet and could not determine the size, number or contents of the cartons. As the car drove off the agents followed it and finally, when they met it, waved it to a stop. As he. got out of the car, petitioner was heard to say, “Hold it; it is-the G’s.” This was followed by, “Tell him he [you] just picked me up.” The agents searched the car, placed the cartons (which bore the name “Admiral” and were addressed to an out-of-state company) in their car, took the merchandise and petitioner and Pierotti to their office and held them for about two hours when the agents learned that the cartons contained[*100] stolen radios. They then placed the men under formal arrest.

The statutory authority of FBI officers and agents to make felony arrests without a warrant is restricted to offenses committed “in their presence” or to instances where they have “reasonable grounds to believe that the person to be arrested has committed or is committing” a felony. 18 U. S. C.' § 3052. The statute states the constitutional standard, for it is the command of the Fourth Amendment that no warrants for either searches or arrests shall issue except “upon probable cause, supported by oath or affirmation,, and particularly describing the place to be searched, and the persons or things to be .seized.”

The requirement of probable cause has roots that are deep in our history. The general warrant,^[1] in which the name of the person to be arrested was left blank, and the writs of assistance, against which James Otis inveighed,^[2] both perpetuated the oppressive practice of allowing the police to arrest and search on suspicion. Police control took the place of judicial control, since no showing of “probable cause” before' a magistrate was required. The Virginia Declaration of Rights, adopted June 12, 1776, rebelled against that practice:

“That general warrants, whereby any officer or messenger may be. commanded to search suspected places without evidence of a fact committed, or to seize any person or persons not named, or whose offence is not particularly described and supported by evidence, are grievous and oppressive, and ought not to be granted.”

[*101] The Maryland Declaration of Rights (1776), Art. XXIII, was equally emphatic:

“That all warrants, without oath or affirmation, to search suspected places, or to seize any person or property, are grievous and oppressive; and all general warrants — to search suspected places, or to apprehend suspected persons, without naming or describing the place, or the person in special — are illegal, and ought not to be granted.”

And see North Carolina Declaration of Rights (1776), Art. XI; Pennsylvania Constitution (1776), Art. X; Massachusetts Constitution (1780), Pt. I, Art. XIV.

That philosophy later was reflected in the Fourth Amendment. And as the early American decisions both before ^[3] and immediately after^[4] its adoption show, common rumor or report, suspicion, or even “strong reason to suspect” ^[5] was not adequate to support a warrant for arrest. And that principle has survived to this day. See United States v. Di Re, 332 U. S. 581, 593-595; Johnson v. Unitéd States, 333 U. S. 10, 13-15; Giordenello v. United States, 357 U. S. 480, 486. Its high water was Johnson v. United States, supra, where the smell of opium coming from a closed room was not enough to support an arrest and search without a warrant.’ It was against this background that two scholars recently wrote, “Arrest on mere suspicion collides violently with the basic human right of liberty.” ^[6]

[*102] Evidence required to establish guilt is not necessary. Brinegar v. United States, 338 U. S. 160; Draper v. United States, 358 U. S. 307. On the other hand, good faith on the part-of the arresting officers is not enough. Probable cause exists if the facts and circumstances known, to the officer warrant a prudent man in believing that the offense has been committed. Stacey v. Emery, 97 U. S. 642, 645. And see Director General v. Kastenbaum, 263 U. S. 25, 28; United States v. Di Re, supra, at 592; Giordenello v. United States, supra, at 486. It is important, we think, that this requirement be strictly enforced, for the standard set by the Constitution protects both the officer and the citizen. If the officer acts with probable cause, he is protected even though it turns out that the citizen is innocent. Carroll v. United States, 267 U. S. 132, 156. And while a search without a warrant is, within limits, permissible if incident to a lawful arrest, if an arrest without a warrant is to support an incidental search, it must be made with probable cause. Carroll v. United States, supra, at 155-156. This immunity of officers cannot fairly be enlarged without jeopardizing the privacy or security of the citizen. We turn then to the question whether prudent men in the shoes of these officers (Brinegar v. United States, supra, at 175) would have seen enough to permit them to believe that petitioner was violating, or had violated the law. We think not.

[*103] The prosecution conceded below, and adheres to the concession here,^[7] that the arrest took place when the federal agents stopped the car. That is our view on the facts of this particular case.- When the officers interrupted the two men and restricted their liberty of movement, the arrest, for purposes of this case, was complete. It is, therefore, necessary to determine whether at or before that time they had reasonable cause to believe that a crime had been committed. The fact that afterwards contraband was discovered is not enough. An arrest is not justified by what the subsequent search discloses, as Johnson v. United States, supra, holds.

It is true that a federal crime had been committed at a terminal in the neighborhood, whisky having been stolen from an interstate shipment. Petitioner’s friend, Pierotti, had been suspected of some implication in some interstate shipments, as we have said. But as this record stands, what those shipments were and the manner in which he was implicated remain unexplained and undefined. The rumor about him is therefore practically meaningless.- On the record there was far from enough evidence against him to justify a magistrate in issuing a warrant. So far as the record shows, petitioner had not even been suspected of criminal activity prior to this time. Riding in the car, stopping in an alley, picking up packages, driving away — these were all acts that- were outwardly innocent. Their movements in the car had-.no tnarkof fleeing men or men acting furtively. The case might bé different if the packages had been taken from a terminal or from an interstate trucking platforim’. But they were not. As we have said, the alley where the packages were picked up was in a residential section.[*104] The fact that packages have been stolen does not make every man who carries a package subject to arrest nor the package subject to seizure. The police must, have reasonable' grounds to believe that the particular' package carried by the citizen is contraband'. Its shape and design might-at times be adequate. The weight of it and the manner in which it is carried' might at times be enough. But there was nothing to indicate that the cartons here in issue probably contained liquor. The fact that, they contained other contraband appeared only some hours after the arrest. What transpired at or after the time the car was stopped- by the- officers is, as. we have said, irrelevant to the narrow issue before us. To repeat, • an. arrest is not justified by what the. subsequent search discloses. Under our system suspicion is not enough for an officer to lay hands on a citizen. It is better, so the Fourth Amendment- teaches, that the guilty sometimes go free than that citizens be subject to easy arrest.

The faict that the suspects were in an automobile is not enough. Carroll v. United States, supra, liberalized the rule governing searches when a moving vehicle is involved. But that decision merely relaxed the requirements for a warrant on- grounds of practicality. It did not dispense with the need for probable caüse.

Reversed.