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Understanding Probable Cause

It's important to know what does and does not constitute PC for any given situation.

May 18, 2010  |  by Devallis Rutledge - Also by this author

A familiar example helps illustrate the distinctions. An officer on night patrol sees a car ahead without lights on, weaving slightly within the lane. This is not PC to arrest, but it is reasonable suspicion to detain for investigation, so a stop is made. The driver might turn out to be sober, driving a rented vehicle and trying to find the light switch, which would explain both of the suspicious circumstances that justified the stop. Or, he might exhibit symptoms of being under the influence and might fail field sobriety tests. These additional facts, added to the erratic driving, would constitute PC to arrest for impaired driving. If his blood alcohol level were subsequently found to be above legal limits and he gave an admissible confession, all of the evidence combined would generally be enough to prove his guilt at trial, beyond a reasonable doubt.

As this example shows, probable cause is a fairly low level of suspicion when compared with courtroom standards of proof. In fact, the Supreme Court has pointed out that an officer can have PC even though the prosecutor might not be able to file charges or convict the person-and even though it might turn out that the person arrested was actually innocent, or that no crime had in fact been committed.

"There is a difference between what is required to prove guilt in a criminal case and what is required to show probable cause for arrest and search." (Brinegar v. U.S.)

"Innocence of the charge is largely irrelevant to [the question of the existence of probable cause]. The Constitution does not guarantee that only the guilty will be arrested." (Baker v. McCollum)

When is PC required?

As discussed above, the Fourth Amendment mandates that warrants be supported by probable cause. The Supreme Court has ruled that in limited circumstances, officers may make warrantless searches and seizures based on PC. These include vehicle searches and public arrests.

Vehicle searches: The court has given two reasons for allowing warrantless searches of a vehicle when police have PC to believe something seizable is inside, and when the vehicle is lawfully accessible (street, driveway, carport, parking lot). First, people have a reduced level of expectation of privacy in vehicles, because vehicles are constantly exposed to public view, are licensed and regulated, and are subject to safety inspections. Second, vehicles are inherently mobile, allowing them to be quickly removed from the jurisdiction while a warrant is being sought. The Supreme Court has therefore dispensed with the warrant requirement for vehicle searches. Such searches may lawfully be made based on lawful access and PC-even if there might have been time to get a search warrant. (Florida v. Meyers)

Public arrests: When officers have PC to believe a particular individual has committed or is committing any public offense, the person may constitutionally be arrested in a public place, without an arrest warrant. (U.S. v. Santana) To make a non-emergency, non-consensual entry into a residence in order to arrest, a warrant is usually required. (Payton v. New York)

When is PC not required?

Although PC is necessary to get a search or arrest warrant, to make a warrantless arrest where permissible, or to search a vehicle for suspected contraband or the fruits, instrumentalities, or evidence of crime, it is not necessary in some situations where many criminal justice professionals wrongfully insist on talking about it. When the law actually sets a lower standard than PC for certain investigative conduct, it can be harmful to police and prosecutors to talk in terms of "probable cause" for acts that do not need that much justification.

For example, there is no constitutional concept called "PC for the stop." A stop is a detention, either of a pedestrian or a vehicle. Detentions are lower levels of intrusion on a person's liberty than arrests. They may be made on the basis of a lower quantity and quality of suspicion. That lower level is called "reasonable suspicion." The Supreme Court has made the distinctions clear:

"The level of suspicion required for a detention [stop] is obviously less demanding than that for probable cause. The police can stop and briefly detain a person for investigative purposes if the officer has a reasonable suspicion that criminal activity may be afoot, even if the officer lacks probable cause." (U.S. v. Sokolow)

"Reasonable suspicion is a less demanding standard than probable cause not only in the sense that it can be established with [less information], but also in the sense that reasonable suspicion can arise from information that is less reliable than that required to show probable cause." (Alabama v. White)

Some other investigative steps may be justified at the lower "reasonable suspicion" level, and should not be misconnected in reports or testimony to the higher PC standard. Examples include weapons pat-downs or "frisks," which require only a reasonable suspicion that the person is armed and dangerous (Terry v. Ohio), safety sweeps of lawfully entered premises based on a reasonable suspicion that a potential assailant may be present (Maryland v. Buie), and making a no-knock entry to serve a warrant based on reasonable suspicion that knocking and announcing would imperil officers, facilitate escape, or permit the destruction of evidence. (Richards v. Wisconsin)

Officers do themselves and prosecutors a disservice when they carelessly speak of their PC for a stop, or a weapons frisk, or a safety sweep. The proper standard is "reasonable suspicion." Sometimes, PC is not required.

Devallis Rutledge is a former police officer and veteran prosecutor who now serves as Special Counsel to the Los Angeles County District Attorney. He is the author of the book "Investigative Constitutional Law" and is POLICE Magazine's resident legal writer.

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Comments (14)

Displaying 1 - 14 of 14

aogden @ 7/25/2010 2:16 PM

I have a question. This was a great article, and really helped define the difference between PC, and reasonable suspicion. However, I was wondering whether or not the suspects permission plays into this at all. For example, if you engage a traffic stop, have the driver at your car, and ask if you can look through their purse or car.....if they say "yes", do you still need PC? Or, does the necessity of PC only come into play when they refuse to give consent?

In short, I guess i am asking this. On that traffic stop, you ask to search the car, and they say "no", IF you have PC, you can do it anyways. If you do not have legitimate PC, then you cannot, correct? But if they say "yes", you can search, even if you may not have enough to claim PC?

CognitiveConsistency @ 7/26/2010 8:34 PM


CognitiveConsistency @ 7/26/2010 8:38 PM


you only need consent in the absence of PC, or to widen the scope of your search. In the absence of PC, either consent to search or a warrant is needed.

aogden @ 7/29/2010 12:31 PM

Thank you! That makes sense. I hope to start the academy within the next 6 months in my local area here, so any insight I can get now, I will gladly take!!

SAM551974D @ 8/22/2010 11:06 PM

The author of this article's books which I have and I read on a regular basis are another tool needed to stay fine tuned and updated...a very good investment indeed! Reading the state's criminal codes and traffic codes are also a plus in my case all 1000 pages worth and most come with the CD for better call up's and refinement...of focus and search! With actual case examples break out the coffee pot...just keep the cup away from pages :-) Good Luck and stay safe!

shirley herbert @ 11/27/2011 10:14 AM

I was required to relinquish my 25 year old firearm because I registered it subsequent the expiration date of the amnesty period given for registering older firearms here in Illinois. I turned the firearm over to officers at my local police station and several police officers recommended that I purchase a Ruger 380 which is a purse size firearm. At the gunshop the seller also indicated to me that the Ruger is a purse size firearm for the purse (I live alone in a rough area in Chicago). Well, Illinois doesn't allow carrying of firearms concealed or not so. If I purchased the Ruger 380 and registed it in accordance with our ordinance, the police would have a record that I own the registered firearm. So, could probable cause (my having a registed purse size firearm that I may be carrying in my purse (which is breaking the law)) be applied by a police officer to search my purse when I am out and about?

leslie smith @ 3/16/2012 8:46 AM

If there is a sexual crime committed and there is an accused, can I seek a warrant from the court to get a DNA sample of another person who I believe is the actual person that did the crime?

e jones @ 6/16/2014 11:35 AM

stopped with no traffic offense commited the officer said he pulled me over because he knew i was driving on a suspended liscence... is this a legal stop????? e.jones

billwhip @ 12/7/2014 8:07 PM

to e jones: If the officer knows you and knows that you have a suspended license, and saw you driving, he has just observed a crime and can therefore stop you. You just gave him the PC to stop you by being in that car driving.

billwhip @ 12/7/2014 8:10 PM

@Shirley Herbert: If you cannot carry concealed in IL, having a gun in your purse is considered carrying concealed and therefore is illegal. If a cop saw you and knew you bought that gun and he knows you do not have a permit, that does not give him cause to stop you and search your purse. but if you commit a crime then yes he would most likely search your purse and if he finds your gun you would be added on another crime.

tim @ 5/17/2015 2:50 PM

@billwhip. simply committing a crime does not give PC for a search of a person. Knowles V Iowa specifically says you cannot search incident to citation. So say Shirley Herbert is trespassing. no other facts present than a simple trespass. Officers would not be able to search her purse and find that weapon without actually taking her to jail for the offense.

jacki @ 6/6/2015 3:05 AM

I was stopped after leaving a convenient store at 2 am with 3 friends. We were walking home with several bags of groceries and I had a shoulder bag I used to carry a few drinks. One officer frisked me then asked to search my bag, he took everything out, opened ever container, make up case, my sons in hailer even. He found a pill case and opened it finding a small empty bag. He said he wasn't going to charge me but I'd have to work the charge off by providing names. He gave me until 6pm that night to call or he'd file the paraphernalia charge. 3 days later I got charges in the mail. He filed them 6 hrs after the search at 8am.

concerned parent @ 12/9/2015 2:43 AM

My son was assaulted in school. When I came to see if he was ok, I was delayed by administrators to see my son. When I asked to see the Dean and or Resource officer I was told they were busy and I needed to make an oppointment. When I finally was able to see the Resource officer, I was shocked when he stated that upon questioning the assailant, he accused my son of robbing his weed that he was selling my son and questioned my son about the incedent without my presence even though I was there waiting. He was not arrested because my refusal to co operate with such an outlandish story coming from another student who assaulted my son. Now he has detained my son on probable cause. Is accusations by an admitted drug dealer credible? And is the Resource officer able to request detention of my son on probable cause? If the assailant is the only witness is he credible?

Advo99 @ 8/19/2016 2:48 PM

Reform - cops require more training, oversight.

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