This paper provides an introductory overview of criminal evidence, covering its major forms—physical, documentary, testimonial, and confessional—and the legal requirements governing admissibility. It examines the Miranda warning, the exclusionary rule applied to illegally obtained evidence, and the concept of probable cause in search and seizure law. The paper then focuses on Illinois v. Caballes (2005), in which the Supreme Court held 6–2 that a drug-detection dog sniff during a lawful traffic stop did not constitute a Fourth Amendment search. The author analyzes the majority and dissenting opinions and argues, with the dissenters, that the absence of specific articulable facts meant police lacked genuine probable cause to extend the stop.
In order to bring an accused person into court, there must be evidence. Without evidence there can be no trial. Criminal evidence can be (1) physical and documentary, (2) testimony of witnesses, or (3) the accused person's confession. Physical evidence includes things like fingerprints, the murder weapon, blood, footprints, tire marks, objects left behind at the crime scene, or DNA. Documentary evidence could be a "paper trail" — receipts, photographs, letters, a diary, drawings, maps, recorded telephone conversations, or video from an in-store camera. Witnesses can be eyewitnesses to the crime or expert witnesses, such as a coroner, psychiatrist, handwriting expert, or social worker.
A confession is only admissible as evidence if the prosecution can show that it was not obtained by any sort of coercion, and that the accused person knew that anything he said could be used against him in a court of law, was aware of his right not to speak, and knew he had a right to have a lawyer present during police questioning. The law requires that these rights be made known to the accused in order to offset the inherently coercive and intimidating atmosphere surrounding a police interrogation when officers suspect an individual of having committed a crime.
If this "Miranda" requirement is not scrupulously adhered to, a waiver may be obtained at trial and the unwarned confession rendered inadmissible as evidence (Dix, 2002).
Evidence may be refutable — that is, it can be shown in court to be untrue — or it may be irrefutable. For instance, for many years fatherhood was considered irrefutable if a child was born during the marriage of the man to the child's mother. Such a man was not entitled to argue in court, for example, that the child had blond hair and blue eyes and stood 6'4" tall while he himself had black hair, brown eyes, and stood only 5'3", and that besides, the child looked exactly like the next-door neighbor. As long as the man was married to the child's mother when the child was born, he was legally the father. In some cases, this has now changed and fatherhood has become refutable since the introduction of DNA testing, which can prove that a man is not the biological father of a child. However, if a man raised the child for several years and then, upon seeking a divorce, attempts to renounce his fatherhood in order to avoid paying child support, a judge is less likely to treat that man's fatherhood as legally refutable (Koenig, 2005).
The police must obtain criminal evidence legally. If evidence is illegally obtained, it will not be admissible in court. For example, if police were to enter and search a suspect's home without a legal warrant and find the murder weapon, that evidence would not be admissible because it was obtained illegally. Suppose a police officer enters the home of a person he suspects of selling counterfeit telephone cards and finds a map showing the location of the cards, then recovers the cards at that location. Because the officer obtained the map through an illegal search, "the phone cards are the fruit of that unlawful search and are therefore inadmissible into evidence" (Understanding Search & Seizure Law, 2005).
Similar rules apply when a police officer makes a traffic stop. In order to search a vehicle, the police must demonstrate probable cause — that is, compelling facts supporting a belief that the driver has committed a crime. The Supreme Court addressed just such a question in a case decided in January 2005.
Roy Caballes was stopped for traveling six miles per hour over the speed limit. The police officer informed Caballes of the reason for the stop and asked to see his license, registration, and proof of insurance. The officer directed Caballes to pull his car onto the shoulder, out of the flow of traffic, and then to sit in the patrol car. The officer told Caballes he intended to issue a written warning.
"Traffic stop leads to dog sniff and marijuana arrest"
"6–2 majority holds dog sniff is not a search"
"Author sides with dissenters on Fourth Amendment grounds"
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