A police investigation involves various moving parts. When an officer tries to get the ball moving and wants to conduct an interview or a search, probable cause is a critical element that the Constitution requires.
What constitutes probable cause and how the police use it are two different prongs in an investigation.
What is probable cause?
If the police suspect someone has or will commit a crime, it alone does not give them carte blanche to do what they please. They need evidence, but how they gather rests mainly on the events leading to their belief. Probable cause allows the police to ask for a search warrant, and to get one from a judge, the officers must show that they have sufficient belief that a citizen has evidence of a crime.
How do the police use probable cause in an investigation?
An officer may stop a car weaving across lane lines because that officer witnesses the driver break a law. The officer may also possess a reasonable belief that the driver is driving under the influence. At that point, the officer begins to question the driver and uses his or her observational skills to determine if there is probable cause to conduct a sobriety test or ask for permission to search the vehicle.
The Fourth Amendment may empower the police to use probable cause to get a warrant, but it ultimately protects the public from illegal search and seizure. A judge may throw out anything obtained when the police do not follow the procedure.