Probable cause means there is sufficient evidence for an arrest. Reasonable suspicion is a step down under the law. An officer may detain and question you because they suspect you are involved in something illegal, but do not have enough evidence to arrest you— yet. In speaking with you, they try to get probable cause.
For instance, you are stopped for a traffic violation. The officer cannot search your car because he or she has no probable cause to do so. Smelling marijuana gives probable cause to search for drugs.
Officers may get you out of the car and talk to you. Reasonable suspicion could turn to probable cause if you say something incriminating or try to get away, for example. If the officer does not have anything but suspicions, however, you must be released.
Let’s say you are walking down a street at night and a burglary just took place nearby. The police have “reasonable suspicion” that you were involved—you are wearing dark clothing, seem nervous and breathless, and your alibi is weak. However, this could change to “probable cause”—perhaps your story that you were in a bar nearby falls apart.
An officer may try to get you to volunteer information. He or she will not tell you that you are free to go, so you must ask: “Am I free to go or are you arresting me?” If they arrest you or take you to the police station, firmly and politely request a lawyer.
It may seem like the officer ‘already knows’ that you have done something illegal, or that he will find where contraband is hidden anyway. This can cause many people to feel like there is no point to objecting to a search or that they should volunteer to tell the officer what they are asking about. Doing this can hurt your case.
Forcing the officer to justify his actions is always preferable over the officer being able to simply state that you consented to the search or told him where he could find the contraband.