Once it is determined that there is a search or seizure for Fourth Amendment purposes, the next inquiry requires a determination regarding whether a warrant is necessary and, if so, what it must contain. The Fourth Amendment provides that “no Warrants shall issue, but upon probable cause, supported by Oath or affirmation.” The Supreme Court has observed “the most basic constitutional rule in this area is that searches conducted outside the judicial process, without prior approval by judge or magistrate, are per se unreasonable under the Fourth Amendment.” Of course, this does not mean that warrants are required for every search. There are exceptions to the warrant requirement which we’ll focus on later in the course. Still, those exceptions are narrow, and the assumption is that all searches should be conducted pursuant to valid search warrants.
To secure a valid warrant, an officer must seek out a detached magistrate or judge and establish probable cause supported by an oath or affirmation. This is usually done through an application for a warrant whereby the facts and circumstances are laid out and the officer signs an affidavit to accompany the application wherein the officer swears or affirms as to the factual basis of the statements in the application. The magistrate or judge who makes the determination regarding probable cause then considers all the statements in the application to determine whether probable cause exists.
While warrants all need to conform to the Fourth Amendment’s minimal requirements, standards regarding the form and content of a warrant are usually established by local law or court rules. The Fourth Amendment provides that warrants must “particularly describe the place to be searched and the persons or things to be seized.” Courts disfavor over-broad and general warrants inasmuch as these are the types of warrants that the framers intended to prohibit when drafting the Fourth Amendment. Although minor mistakes do not necessarily invalidate warrants, the descriptions of the places to be searched and the persons or things to be seized must be sufficient to leave “nothing . . . to the discretion of the officer executing the warrant.”
Probable cause, like “search or seizure,” is a phrase that is explicitly mentioned by the Fourth Amendment. Probable cause is the standard that must be met for any warrants to issue and for any searches or seizures to be executed. There are two definitions of probable cause – the level of probable cause necessary for a search and that necessary for an arrest.
Let’s start with the probable cause necessary to justify a search. Probable cause is determined based on the totality-of-the-circumstances test, which means that the court will consider all the factors present and known to the officer at the time of the search or seizure.
Probable cause to search exists when the facts and circumstances within the officers’ knowledge and of which they have reasonably trustworthy information are sufficient in themselves to warrant a man of reasonable caution in the belief that an item subject to seizure will be found in the place to be searched. The search cannot be based on just a “hunch.”
Assume an officer notices a car speeding down a residential road at 2:00 AM. After the officer pulls the vehicle over, the officer notices that the driver has red and glassy eyes. The officer orders the driver out of the car, searches the car, and finds a large quantity of marijuana. A court, after considering the totality of the circumstances, would likely hold that the facts known to the officer at the time of the search did not rise to the level of probable cause and the search was based on a hunch. As such, a court would likely conclude such a search was illegal under the Fourth Amendment.
However, assume the officer observes the car weaving back and forth on the street at 2:00 in the morning. After pulling the car over, the officer approaches the driver’s side window and notices the occupants are very nervous. The officer asks the driver to lower the window and when the driver does, he notices a foggy substance coming out which appears to be marijuana smoke. While speaking with the driver, the officer notices a Ziploc bag of pills, a pipe and a bag with an unidentified white powdery substance lying next to the front-seat passenger’s feet. It seems clear that in this case there is probable cause to search the car.
However, it is not always as simple as choosing between one of these scenarios. Facts can range from clear and obvious to obscure and everything in between. Probable cause to arrest is a separate analysis. It exists where the facts and circumstances within the officers’ knowledge and of which they have reasonably trustworthy information are sufficient to warrant a man of reasonable caution in the belief that an offense has been or is being committed by the person arrested. There must be facts to show that the person committed or is in the process of committing a crime.
Going back to the officer who had probable cause to search the car, strong evidence of illegal drugs in the case is probable to arrest at least the driver and any occupant who apparently has possession of the illegal substances.
The Exclusionary Rule and Fruit of the Poisonous Tree
The exclusionary rule dictates that evidence seized illegally by a government agent is inadmissible at trial. The rationale behind this rule is deterrence. The idea is that if the evidence is excluded at trial it will force government agents to follow the law and will deter them from violating the Fourth Amendment.
The exclusionary rule is a relatively new doctrine in American jurisprudence. Its roots were in a 1914 case, Weeks v. United States. Because that case involved the federal government, for many years it was not clear whether the exclusionary rule applied to the states. However, in 1961, the Supreme Court decided Mapp v. Ohio, extending the exclusionary rule to the states through the Due Process Clause of the Fourteenth Amendment. Thus, regardless of how incriminating the evidence is, evidence seized illegally by law enforcement may not be used against any state or federal criminal defendant.
Moreover, once evidence is excluded, the fruit of the poisonous tree rule requires that all evidence that was obtained because of the illegally seized evidence is also inadmissible at trial. In Silverthorne Lumber Co., Inc. v. United States, the Supreme Court explained that the Fourth Amendment prohibited the Government from gaining any advantage through illegally-obtained evidence.
The fruit of the poisonous tree rule can sometimes mandate harsh and even incongruous results. Take, for example, the case of a police officer who illegally searches a person’s trunk and finds cocaine. The officer arrests the driver and, while conducting a search incident to the arrest, finds an illegal handgun in his pocket that is later tied to a murder. He then gets a warrant and searches the driver’s house, where he finds two dead bodies in the basement, a stash of child porn on his computer and 12 kilograms of methamphetamine behind the peanut butter in the kitchen pantry. After being arrested, the suspect confesses to 3 murders, drug trafficking, running a child prostitution ring and to being an all-around bad guy.
All this evidence would be excluded under the fruit of the poisonous tree rule, including the confession because none of this would have been obtained but for the illegal search of the trunk. It all grew from the poisoned tree of the initial illegal search. Unless the government can find other, clean evidence, this really, really, bad guy will walk.
Exceptions to the Exclusionary Rule
Over the years, the Supreme Court has carved out exceptions to the exclusionary and fruit of the poisonous tree rules, largely to mitigate their harsh effects. In United States v. Leon, the Supreme Court carved out a “good faith exception” to the exclusionary rule. It allows the prosecution to use evidence obtained from an invalid warrant so long as that warrant was executed by the police in good faith. Keep in mind that the exclusionary rule is meant as a deterrent – to convince police officers to respect the Fourth Amendment rights of suspects. You cannot deter good faith mistakes. If the police acted based on what they thought was a valid warrant, there would be no point in applying the exclusionary rule.
However, where the “magistrate or judge in issuing a warrant was misled by information in an affidavit that the affiant knew was false or would have known was false except for his reckless disregard for the truth,” then suppression would be required. In such case, there is no true “good faith.” Moreover, the good faith exception applies only where police held a (supposedly) valid warrant. If police make a warrantless search under what they believe is an exception to the warrant requirement, or where police conduct a search and they (honestly) believe that no warrant is required, there is no applicable exception and the exclusionary rule must be applied.
Additionally, the Supreme Court has established the impeachment exception to the Exclusionary Rule. Even if evidence is excluded, if the defendant takes the stand in a criminal case and lies on the stand, the prosecution can admit the illegally obtained and excluded evidence to impeach the defendant. So, for example, if the police illegally seize the defendant’s cocaine and the defendant testifies that he’s never possessed cocaine in his life, the prosecution can introduce the seized cocaine to impeach that statement. However, it should be noted that this applies only to impeach the defendant, not to impeach other witnesses who may testify on the defendant’s behalf.
Exceptions to the Fruit of the Poisonous Tree Rule
Where illegally obtained evidence leads to the discovery of other evidence, the fruits of the poisonous tree can still be admissible if there was an independent source of that evidence. So, if, based on evidence found during an illegal search, police find a trove of guns or narcotics, but police can show that another witness had told, or was about to tell, police of the location of the same trove, it is admissible. Here, the evidence would have been discovered even without the illegal search and so it is not considered tainted evidence. 
The next exception is the rule of attenuation, which allows a court to admit otherwise inadmissible evidence when it is sufficiently separated from the initial illegal discovery of the evidence. The idea is—somewhat similar to the independent source doctrine—that the attenuation dissipates the illegal taint. In Brown v. Illinois, the Supreme Court laid out three factors which courts should consider when determining whether the violation is sufficiently separated from the initial taint: (1) the length of time between the illegality and the seizure of evidence; (2) the presence of additional intervening factors; and (3) the degree and purpose of the official misconduct.
Suppose an officer stops a person with no basis, constituting an unlawful seizure. Following the stop, the officer runs the driver’s information. In the course of this search, he pulls up an outstanding arrest warrant. The officer arrests the person, searches him and finds a bag of cocaine, a wad of cash and a small ledger with what looks to be customer information pertaining to drug sales.
Notwithstanding the initial illegal stop, a court could find that the outstanding arrest warrant attenuated the connection between the unlawful stop and the evidence seized incident to arrest. The court would consider the intervening factor of the outstanding warrant (which was obviously not a product of police misconduct) and may also consider whether the police officer originally made the stop with some colorable basis to believe it was legitimate. If the initial stop was an honest mistake, the court could conceivably rule that the illegal stop was far enough removed from the ultimate search to avoid applying the fruit of the poisonous tree rule, even though the stop was certainly the “but-for” cause of finding the cocaine.
Finally, another exception to the Exclusionary Rule is what is called the inevitable discovery exception. The Court established the inevitable discovery exception in 1984 in Nix v. Williams. The inevitable discovery exception allows evidence to be used if the government would have found the evidence regardless of the illegality.
As an example, assume officers pulled over a defendant for driving under the influence and placed him under arrest. Before taking the defendant to the station, the officers illegally opened the trunk and found illegal guns. Although the officers illegally opened the trunk, the officers were going to tow the vehicle down to the station and perform an inventory search to secure the defendant’s belongings anyway. As such, the guns would have been discovered in short order even without the illegal search.
In the next Module, we will begin our discussion on the exceptions to the warrant requirement and where warrants are unnecessary for proper searches.
 U.S. Const., amend IV.
 Coolidge v. New Hampshire, 403 U.S. 443, 444-45 (1971) (internal quotation marks omitted).
 John W. Hall, Jr., 5 Search and Seizure § 53.03 (LEXIS 2017).
 U.S. Const., amend IV.
 “Article: I. Investigations and Police Practices,” 43 Geo. L.J. Ann. Rev. Crim. Proc. 3, 31 (2014).
 Andresen v. Maryland, 427 U.S. 463, 480 (1976).
 Kit Kinports, “Probable Cause and Reasonable Suspicion:Totality Tests or Rigid Rules?” 163 U. Pa. L. Rev. 75, 75-76 (2014).
 See Carroll v. United States, 267 U.S. 132, 149 (1925).
 Brinegar v. United States, 338 U.S. 160, 175-76 (1949).
 Mapp v. Ohio, 367 U.S. 643, 648 (1961).
 Weeks v. United States, 232 U.S. 383, 398 (1914).
 Mapp v. Ohio, 367 U.S. 643, 655-56 (1961).
 Silverthorne Lumber Co., Inc. v. United States, 251 U.S. 385, 391-92(1920); Robert M. Pitler, “The Fruit ofthe Poisonous Tree Revisited and Shepardized,” 56 Cal. L. Rev. 579, 589 (1968).
 Silverthorne Lumber Co., 251 U.S. at 391-92.
 United States v. Leon, 468 U.S. 897, 922 (1984).
 Id. at 923.
 See Harris v. New York, 401 U.S. 222, 226 (1971).
 Jeffrey M. Bain & Michael K. Kelly, “Fruit of the PoisonousTree: Recent Developments as Views Through Its Exceptions,” 31 U. Miami L. Rev. 615, 622-25 (1977).
 Heather A. Jackson, “Arizonav. Evans: Expanding Exclusionary RuleExceptions and Contracting Fourth Amendment Protection,” 86 J. Crim. L. & Crim. 1201, 1209 (1996).
 Brown v. Illinois, 422 U.S. 590, 603-04 (1975).
 See Utah v. Strieff, 136 S. Ct. 2056, 2062 (2016).
 Nix v. Williams, 467 U.S. 431, 443-44 (1984).
 See id. at 443-44; Jackson, supra note 19, at 1206.