He may be guilty but without probable cause for the arrest the evidence will be suppressed
One of the great cases for criminal defense lawyers is Dunaway v. New York, decided by the United States Supreme court in 1974 and printed officially at 441 U.S. 200 and more conveniently (but less officially) at 99 Sup. Ct. 2248. The case establishes a constitutional right of criminal defendants regarding evidence seized by law enforcement authorities and mandates that the prosecutions' evidence will be suppressed when it resulted from a defendant's arrest without probable cause.
Before we discuss the case, let us define the operative terms.
"Probable cause" is what the police need to arrest a person. While it's precise definition will vary occasionally as a function of the court interpreting the term, it basically means that the police must have a reasonable grounds for their opinion that an alleged perpetrator has committed a crime before they can arrest the villain. Even though the defendant may be guilty as sin, if they cannot enunciate (or at least fabricate) probable cause, any evidence seized as a result of that arrest will be suppressed.
The next term is "suppression." In order to admit evidence against a defendant at trial, the evidence must be admissible. That means that it must have been seized in accordance with the rules of law that deal with seizures of evidence. Generally, when evidence is seized from a defendant, I and every other competent criminal defense lawyer make a "Dunaway" motion asking that the evidence not be allowed into evidence because my client was arrested without the arresting officer having probable cause to make the arrest. When the court issues a ruling that the arrest was in fact made without probable cause, the evidence is not allowed to be used by the prosecution. In lawyer language, the evidence is suppressed.
Poor Mr. Dunaway. Apparently, after talking to a jail inmate, a Rochester, New York police detective ordered that Mr. Dunaway be arrested, even though he had no legally valid reason to make the arrest. After being given the warnings required by Miranda v. Arizona,
Mr. Dunaway waived counsel and eventually made statements and drew sketches that incriminated him in the crime.
I will spare you the long and tortured history of the case. After myriad waffling on the parts of numerous courts, the matter eventually appeared on the calendar of the Unite d States Supreme Court.
The United States Supreme Court issued a ruling that established the principal that in order for either evidence seized from a defendant or statements made by her to be admitted into evidence, the arrest must have been lawfully made. If the arrest was not made after "probable cause," then the arrest will be denominated as illegal and any evidence seized as a result of that arrest will be suppressed and not allowed to be used in evidence against the defendant.
Recently, I had just such a case. My client, a young black male, was driving an extremely fancy and expensive vehicle with tinted windows. The cops, acting on racial stereotypification, decided to stop him and see what happened. They found all sorts of contraband. It was a display of excellent police work. However, since there was no probable cause for the arrest, the evidence had to be suppressed. I suppose the police can console themselves with the thought that they got quite a few weapons and a major stash of drugs off of the streets.