Most people know that police can search a home or business if they have a search warrant. The process of obtaining such a warrant may not be as familiar. It all boils down to probable cause and whether the police have probable cause to get a search warrant. If a judge believes there is probable cause to search for items relating to criminal activity, then he or she will sign and issue the search warrant.
If, however, a police officer or other government actor illegally searches for evidence of a crime, then the "exclusionary rule" kicks in. This rule keeps the prosecution from presenting at trial any evidence that was discovered in the illegal search. Many exclusionary rule opponents believe that this is just a way to let criminals off on a technicality.
There's another issue with illegal searches. Evidence may be gathered during the illegal search that will lead police to find more evidence. That new evidence cannot be used at trial, either. This rule is known as the "fruit of the poisonous tree." It keeps police from using an illegal search to find additional evidence to use against a defendant.
The prosecution may not be able to use evidence collected because of an illegal search to obtain a conviction that they might be able to use in other parts of the trial. For example, the evidence might be used in immigration or civil cases. The prosecution can attack a witness' credibility with the evidence, but only in certain circumstances. Also, a judge could look at the evidence during sentencing.
While police cannot perform an illegal search and use seized items to get a criminal conviction, it can sometimes be difficult to prove the search was illegal. That's why it's important to have an experienced criminal defense attorney by your side.
Source: FindLaw, "Search Warrant Requirements," accessed July 17, 2015