Criminal Procedure


Under the Fourth Amendment, a search occurs when government action obtains information either by committing a trespass (from Jones) or in violation of the reasonable expectation of privacy of the one being searched (from Katz).

  • Applying a GPS tracker to someone's car constitutes a trespass and therefore a search. Jones.
Reasonable Expectation of Privacy

Since Katz, a search is said to occur under the Fourth Amendment when the government violates a person's reasonable expectation of privacy. This comprises two parts:

  1. The person searched must have exhibited an actual subjective expectation of privacy.
  2. Society must be prepared to recognize that this expectation is objectively reasonable.
Open Field Doctrine

The open field doctrine, as laid out in Oliver, states that searching an open field is not a search within the meaning of the Fourth Amendment because, even though it would be a trespass at common law, there is no reasonable expectation of privacy in an open field.

Land is not an open field if it is within the curtilage of the home.


The curtilage of a home is the land immediately surrounding it.

United States v. Dunn laid out four factors to determine where land is part of the curtilage or an open field:

  1. The distance of the area from the home
  2. Whether the area is within an enclosure surrounding the home (I.e., if it's in a fence)
  3. The nature of the use to which the area is put
  4. Steps taken to protect the area from observation

The basic analysis behind this is whether the land is so intimately tied to the home that it should be placed under the home's Fourth Amendment protection.

Aerial Search

Aerial searches are not searches under the meaning of the Fourth Amendment to the Constitution of the United States as long as the police are in a lawful airspace because there is no reasonable expectation of privacy from being viewed from such places since anyone can fly a plane there.

Sense-Enhancing Technology

Sense-enhancing technology constitutes a search under the Fourth Amendment if the technology is not generally available to the public and it reveals information that one would reasonably expect to be private.

Thermal Imaging

According to Kyllo v. United States, taking a picture of the infrared light being emitted from someone's home is considered a search under the Fourth Amendment because it can reveal the heat being generated in various rooms of the house, which can reveal details about the interior of the home, which has the greatest expectation of privacy, and because when Kyllo v. United States was decided, infrared cameras were much more expensive than they are today.


Generally, there is no reasonable expectation of privacy in one's trash, and therefore a search of trash would not be a search under the meaning of the Fourth Amendment.

Factors such as the location of one's trash can show a reasonable expectation of privacy however.

Vehicles have lower expectations of privacy because they are made to transport one through the public and because they have windows allowing anyone to see inside.

There is no reasonable expectation of privacy in the phone numbers one dials, only in the contents of such calls.


Dogs can be used to search during a lawful traffic stop without it constituting a search under the Fourth Amendment to the Constitution of the United States because there is no reasonable expectation of privacy in the smells that one gives off.

A dog alerting during a search gives an officer probable cause.

Dogs being used within the curtilage of one's home is a search however because, even if though people generally have a license to enter premises for certain purposes like going up to the front door, bringing dogs to sniff is exceeding the police's license to enter and is therefore a trespass.

For a Fourth Amendment search to be constitutionally permissible, it must be reasonable. This generally means that it must be conducted pursuant to a valid search warrant based upon probable cause. Warrantless searches are per se unreasonable.

Search Warrant

A warrant is a writ by a judge or magistrate authorizing police to conduct an arrest or search.

A valid warrant must be based on probable cause.

Probable Cause
Brinegar v. United States

Probable cause exists where "the facts and circumstances within their [the officers'] knowledge and of which they had reasonably trustworthy information [are] sufficient in themselves to warrant a man of reasonable caution in the belief that" an offense has been or is being committed.

An officer's subjective intent or belief is irrelevant in determining probable cause. Only what a reasonable officer would believe matters.

A mistake of fact or law is okay as long as it is reasonable for an officer to make.

Probable cause requires more than just reasonable suspicion, but it does not require evidence that would justify a conviction.

Anonymous Tip

Anonymous tips alone cannot establish probable cause, but they can when combined with the totality of the circumstances. Especially important in such determinations are whether the informant is credible—likely to be telling the truth—and reliable—likely to have knowledge. Thus, the veracity of the informant and his basis of knowledge should be analyzed.

Note that just because an anonymous tip turned out to be true does not mean that it was credible. The totality of the circumstances must establish probable cause before conducting the search, not after. This can be done by corroborating parts of a tip that do not require infringing upon one's reasonable expectation of privacy.

General warrants are not constitutional. They must be supported by oath or affirmation and particularly describe the place to be searched and the persons or things to be seized.

  • A warrant must relate to a specific crime.
Executing a Warrant

Officers can detain and question people in the immediate vicinity while executing a warrant for the purpose of ensuring the safety officers and others.


Generally, officers must knock and announce their presence when entering a home while executing a warrant. However, courts apply a reasonableness test to this requirement which allows exceptions for several reasons where knocking and announcing would cause more harm than good:

  • Exigent circumstances
  • HP
  • Officer safety
  • Preventing the destruction of evidence

Note that the exclusionary rule does not apply to evidence obtained in violation of the knock-and-announce rule anyway.


If a mistake is made in executing a search warrant, the search will still be valid as long as the police acted objectively reasonably in their efforts to identify the place to be searched.

Courts are very lenient on mistakes as long as the officers acted in good faith.

Exceptions to the Warrant Requirement
  • Exigent Circumstances
    Hot Pursuit

    Searches made in hot pursuit of a suspected felon are valid when speed is essential for the safety of officers or others. The hot pursuit exception does not permit police to enter a home without a warrant for a routine arrest however, even if they have probable cause that one committed a felony.


    Police may enter a home without a warrant when they have an objectively reasonable basis for believing that an occupant is seriously injured or immediately threatened with serious injury.

    Destruction of Evidence

    Warrantless entry of a home to prevent the destruction of evidence is reasonable under the Fourth Amendment—and thus allowed—when the police's conduct was reasonable.

    The police's conduct is reasonable when the police did not create the exigency by engaging in or threatening to engage in conduct that violates the Fourth Amendment.

    Drivers may be required to submit to a breathalyzer test without a search warrant to prevent the destruction of evidence but not a blood test because that would be too intrusive.

  • Plain View

    Police may seize evidence that is in plain view without a warrant.

    For the plain view exception to apply, it must be immediately apparent that the item is illegal from a location where the officer is lawfully located.

    Although it is called the "plain view" doctrine, an officer may seize evidence he detects with any of his senses. Most notably, it permits the seizure of evidence felt during a stop-and-frisk that is immediately determined to be illegal.

  • Automobile Exception

    One of the most important exceptions to the warrant requirement is the automobile exception, permitting officers to search anywhere inside a movable vehicle (even if it is not immediately mobile) without a warrant as long as there is probable cause to believe that that area of the vehicle could contain evidence of a crime.

    This is justified because vehicles have lower expectations of privacy because they are made to transport one through the public and because they are subject to many regulations.

    The automobile exception applies not only to the passenger compartment. It also permits searches of the trunk or containers inside a vehicle provided that the police have probable cause that contraband is contained therein.

    • If a container that police have probable cause to search is put into a vehicle, police then have probable cause to search both the vehicle and the container.
    • This includes passengers' items if police have probable cause for them too, such as by virtue of them being in the area of control of a suspect.

    The automobile exception does not permit police to enter the curtilage of one's home to conduct a warrantless search of his automobile.

  • Search Incident to Arrest

    A warrantless search without consent conducted when a lawful arrest is made is reasonable within the area of the arrested person's immediate control. This is true regardless of the crime arrested for, but the person must actually be arrested.

    The purposes of this exception are officer safety and the preservation of evidence.

    Police cannot search a cell phone incident to arrest.

    Police have the right to search the passenger compartment of a vehicle incident to the arrest of an occupant of the vehicle if he has access to it or if the police reasonably believe that the vehicle contains evidence of the crime arrested for.

  • Inventory Search

    Routine inventories subject to standard police procedure are reasonable under the Fourth Amendment.

    Inventory searches are permitted for the purposes of:

    • Protecting the confiscated property
    • Protecting the police from liability
    • Protecting the police from dangerous items

    Inventory searches must be conducted for these purposes to be valid under the Fourth Amendment. They cannot just be to look for evidence of a crime.

    Inventory searches do not need probable cause.

  • Protective Sweep

    Protective sweeps are allowed as long as an officer has a reasonable belief based on specific and articulable facts that the area harbors an individual posing a danger to those on the scene. Evidence found during such a sweep is admissible.

  • Special Need Search

    Special need searches are searches with an immediate objective of a special need separate from tradition law enforcement purposes. They are thus permitted without a warrant or probable cause as long as the government interest outweighs the individual's privacy interest. (This is a two-part test.) In weighing an individual's privacy interest, both the character of the intrusion and the nature of the intrusion must be considered.

    Administrative Search

    Generally, routine administrative searches require either a warrant, consent, or another exception to the warrant requirement. Absent such requirements, administrative searches must give an opportunity for precompliance review before a neutral decisionmaker unless it is a closely regulated business.

    In applying for a warrant, for an administrative search, probable cause exists if reasonable legislative or administrative standards for conducting such inspection are satisfied for the dwelling. Such standards are reasonable if they are based on the passage of time, the nature of the building, or the condition of the area.

    Warrantless searches are allowed for closely regulated businesses however as long as three requirements are satisfied:

    1. There must be a substantial government interest that informs the regulatory scheme pursuant to which the inspection is made.
    2. The warrantless inspections must be necessary to further [the] regulatory scheme.
    3. The statute's inspection, in terms of the certainty and regularity of its application, must provide a constitutionally adequate substitute for a warrant.
      • It must:
        1. Advise the property owner that the search is being made pursuant to the inspecting officers and has a properly defined scope
        2. Limit officer discretion
    Closely Regulated Business

    The Supreme Court has recognized four industries as being closely regulated:

    • Liquor sales
    • Firearms dealing
    • Mining
    • Running an automobile junkyard
    Border Search

    The government has authority under the Fourth Amendment to conduct suspicionless searches at the border, including removing the gas tank from a car, based on its interest in protecting the border.

    This also allows mail entering the country to be inspected without a warrant, as long as there is reasonable cause to suspect that there is something illegal therein.

    Reasonable suspicion is sufficient to detain someone at the border while waiting for a warrant to conduct a search.


    While a Fourth Amendment seizure occurs when a vehicle is stopped at a checkpoint, there is no Fourth Amendment violation for sobriety checkpoints or checkpoints based on exigency (catching a dangerous criminal) because the main objective is public safety, not general crime control.

    The government's interest in the checkpoint must be balanced against its invasion on the motorist.

    School Search

    Searches in schools only need reasonable suspicion of evidence of violating the law or a rule of the school.

    Measures used in such searches must be "reasonable related to the objectives of the search and not excessively intrusive in light of the age and sex of the student and the nature of the infraction."

    Government Employment Search

    Neither a warrant nor probable cause is needed for a search of a government employee's workplace and work phone as long as it is motivated by a "legitimate work-related purpose" and it is not "excessively intrusive".

    Drug Testing

    Random drug testing is permissible for student athletes and participants in extracurricular activities because there is a lower expectation of privacy inherent in participating in such activities.

    Jail Search

    Strip searches of people entering jail are permissible as long as they are reasonably related to legitimate "penalogical interests".


    DNA may be collected from people validly arrested as part of a routine booking procedure and used to investigate.

    Probation Search

    A person on probation has a lesser expectation of privacy and thus may be searched without a warrant and based only on reasonable suspicion.

    Parole Search

    A person on parole has a lesser expectation of privacy and thus may be searched without a warrant or any level of suspicion.