If There is No PC...
When you have reasons to suspect that a search would disclose some evidence but those reasons don't amount to PC, there may be other ways to conduct a constitutional entry and search:
Consent . "In situations where the police have some evidence of illicit activity, but lack probable cause to arrest or search, a search authorized by a valid consent may be the only means of obtaining important and reliable evidence." (Schneckloth v. Bustamonte)
Probation or Parole Term. In many jurisdictions, probationers and parolees are subject to a term of warrantless, suspicionless searches and seizures. This may provide a lawful basis for entry and search. (Samson v. California)
Incident to Arrest. At or near the time and place of arrest, you may search the arrested person. (U.S. v. Robinson) If the arrestee was the recent occupant of a vehicle, the passenger compartment of the vehicle may be searched. (Thornton v. U.S.) Where the arrest takes place within premises lawfully entered, you may search the area within the arrestee's immediate control (Chimel v. California), and look into immediately adjoining spaces where potential assailants might be. (Maryland v. Buie)
Officer Safety. You may make a weapons frisk of a detainee when there is articulable suspicion he is armed and dangerous (Terry v. Ohio), a safety sweep of lawfully entered premises with articulable suspicion an assailant may be present (Maryland v. Buie), and a routine shakedown of jail or prison cells. (Block v. Rutherford)
Booking Search. Once a prisoner is lawfully in custody at a police station or jail, he and his property can be thoroughly searched for administrative and security purposes. (Illinois v. Lafayette)
Inventory. A lawfully impounded vehicle and its contents can be inventoried according to standard procedure. (Colorado v. Bertine)
If There is No Time...
Assuming there is PC for a search, the next inquiry is, "Do I have time to get a warrant?" The Supreme Court has recognized that in exigent situations, the fact that you don't have time to get a warrant will excuse the warrant requirement to the extent of allowing immediate entry or search to neutralize the exigency. (Mincey v. Arizona) The following exigencies have been recognized:
Rescue. If someone inside premises is in need of immediate help, or if a suspect or his property must be searched to reveal the location of a kidnap victim in distress, you may take necessary steps to neutralize the exigency without waiting for a warrant. (Brigham City v. Stuart)
Imminent Substantial Property Damage. No warrant is needed before activities aimed at countering an immediate threat to property, such as a fire or a burglary in progress. (Michigan v. Tyler)
Imminent Destruction of Evidence. Warrantless entry and search for destructible evidence are constitutional if occupants are aware of law enforcement knowledge of it and are likely to dispose of it quickly. (Ker v. California) This same rationale allows warrantless seizure of "evanescent evidence" (such as blood-alcohol samples) based on probable cause. (Schmerber v. California)
Fresh Pursuit. When the suspect in a recent crime of violence flees from police apprehension and seeks refuge inside private premises, there is no need to interrupt continuous pursuit to seek a warrant. (Warden v. Hayden)
Preventing Escape. If you lawfully attempt a detention or arrest in public and the person retreats into private premises, you may enter and search for him to complete the arrest. (U.S. v. Santana)
Public Safety Threat. No warrant is needed when immediate action is required to protect public safety (bomb threats, hazmat, mass shootings, etc.) (Cady v. Dombrowski)
Whichever exigency permits initial warrantless entry and search, your activity is limited to steps to neutralize the exigency. Once the exigency ends, any further search must be by warrant, consent, or some other exception. (Michigan v. Clifford)
With PC and time...
One recognized exception to the warrant requirement relates to "fleeting targets," such as cars, vans, trucks, buses, RVs, boats, planes, and trains. (California v. Carney) Because of their inherent mobility and diminished expectation of privacy, the court has dispensed with the warrant requirement for fleeting targets. Assuming lawful access, you may search any part of a fleeting target that you have PC to believe conceals something seizable. (U.S. v. Ross) A "fleeting targets" search requires no warrant, even though the vehicle is in exclusive police custody and there is ample opportunity to obtain a warrant. (Michigan v. Thomas)
For any search, whether of a house, a car, or a container, if you have probable cause and time to get a warrant, it's a good idea to seek one. Even though one of the exceptions might apply, you will generally reduce the risks of both suppression of evidence and civil liability if your entry and search are judicially authorized. And if there is no identifiable exception, there is no lawful way to conduct a search except with a warrant—which brings us back to the rule-of-thumb: try to get a warrant whenever possible.
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