What legal authorization is necessary to start an investigation?

A warrant is often necessary to start an investigation because it is a legal document issued by a judge or magistrate that authorizes law enforcement to conduct a search or seizure of property, or to gather evidence related to a crime. This requirement ensures that there is probable cause to believe a crime has been committed and protects individuals' Fourth Amendment rights against unreasonable searches and seizures.

While a subpoena is another form of legal authorization, typically used to compel the production of documents or testimony in conjunction with a legal proceeding rather than initiating an investigation, it is not often the primary mechanism to start an investigative process. An internal review may refer to processes within an organization that does not generally involve law enforcement oversight or legal authority to gather evidence in the context of criminal investigations. Therefore, in many investigative scenarios, particularly those involving law enforcement and the need for adherence to legal standards, a warrant is the most vital component for moving forward legally and ethically with the investigation.