Final answer:
A criminal investigation requires probable cause, an arrest warrant, or a criminal matter to commence. Probable cause is essential in detaining suspects, as seen in the example of individuals suspected of armed robbery. The Fourth and Sixth Amendments to the U.S. Constitution outline the rights and legal processes necessary for a lawful criminal prosecution.
Step-by-step explanation:
A criminal investigation cannot begin without the presence of one of three essential elements: probable cause, an arrest warrant, or a criminal matter that requires investigation. If we refer to the scenario of two individuals apprehended on suspicion of an armed robbery, the police must have probable cause to detain them. Probable cause would mean that a reasonable person would believe that a crime was committed and that the individuals in question were involved in said crime. In the context of criminal justice and the rights of the accused, the requirement for probable cause is linked to the Fourth Amendment to the United States Constitution, which protects against unreasonable searches and seizures. Moreover, the procedural safeguards outlined in the Sixth Amendment guarantee the accused's right to a speedy and public trial, the right to be informed of the nature and cause of the accusation, and the right to obtain witnesses in their favor. These constitutional protections reflect the elements that must be established before a suspect can be tried in court. For the police to proceed with their investigation against individuals suspected of armed robbery, they would indeed need direct evidence or an arrest warrant based on probable cause.