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In the vast majority of cases where an arrest is made or a search is conducted by police officers, those officers must have probable cause to take action. The Fourth Amendment first established this idea in U.S. law, and it was done to protect people from the authorities; the police cannot simply pick someone up for no reason and then search for a reason to arrest him or her after the fact. They need to have just cause to make that arrest, and they will have to show that they did if the case goes to court.
The problem with probable cause is simply that the idea is a bit open-ended, and open to interpretation. For example, if the police believe that someone committed a crime, and if that belief is reasonable and makes sense, the court will often determine that they had probable cause and that all actions taken afterward -– such as the arrest itself -– were lawful. Exactly what the police need to see to reasonably assume that a crime occurred is different from case to case, so there is no way for this to be established in a more solid fashion.
For instance, police may see someone fleeing from a residence and holding something that appears to be a weapon, and they could then make an arrest. They have to react quickly, but there must be a reason for that reaction. The person must be acting suspiciously, or fit the description for a crime that was called in. If that same person was merely walking along, doing nothing out of the ordinary, police would not be within the law to arrest him or her and search for a weapon or stolen property. They cannot randomly accost citizens in this way. Of course, exactly what makes a specific officer wary of an individual is impossible to define, so this is something of a gray area within the law.
This especially becomes important if the police do not have a warrant and they make an arrest, anyway. One of the first steps that they will take after this happens is to go before a judge or similar authority to show that they had probable cause and that, as such, they did not need to get a warrant. This happens in many cases where officers believe that they have caught someone at the scene of a crime, as there may not be time to get a warrant and follow the necessary legal steps, so they take action based on the visible evidence and their feelings about the situation.
For instance, if officers see someone steal a car, they do not need a warrant to apprehend the person. Probable cause has already been established since they witnessed the crime.
Probable cause also comes into play when police are requesting a warrant to carry out a search of a building. They must show that there is reason to conduct the search. They cannot simply search any home or commercial building, without a reason to do so, looking for infractions. They must show that there is a reason for the search and only then can they go in and look for evidence. Once again, probable cause in these situations is open to interpretation, but officers who carry out a search and find the evidence they are looking for may have to prove that they had reason to do so in court. If they did not have probable cause and were not able to obtain a warrant, the evidence will be thrown out.
This article is intended to be helpful and informative. But even common legal matters can become complex and stressful. A qualified criminal defense lawyer can address your particular legal needs, explain the law, and represent you in court. Take the first step now and contact a local criminal defense attorney to discuss your specific legal situation.