During a sexual assault investigation, law enforcement will gather all available evidence. Law enforcement and the prosecution will then evaluate the totality of the evidence and this assessment will guide their decision whether to charge an alleged offender with criminal charges and what those charges will be.

Evidence comes in many forms. Here are some examples:

• statements, both written and oral, from the victim and witnesses
• physical evidence (e.g., fingerprints, DNA, torn clothing)
• documentary evidence (e.g., medical records documenting injuries sustained by the victim during the assault)
• statements from the perpetrator.

Evidence is gathered in variety of ways, including:

• interviewing people with knowledge of the crime
• receiving items of evidence or information from the victim or witnesses
• obtaining search warrants to seize additional evidence. A search warrant is an order signed by a judge that gives law enforcement permission to search for evidence in a location that they otherwise would not have lawful access to, and to take possession of such relevant evidence if they find it.

For physical evidence to be introduced in court proceedings, the party who introduces the evidence must be able to show that it has not been altered. This is done by establishing a chain of custody for the evidence. Chain of custody means an unbroken trail of accountability; each professional who handled the evidence accounts for the time during which they possessed it to establish that there was no opportunity for the evidence to be tampered with, altered, or destroyed.

Example: Police officers respond to the scene of a robbery and are informed by the victim that the defendant brandished a knife and stole the victim’s property. The police find a knife on the ground and the victim identifies it as the one the defendant held. Police officers take possession of the knife, package it, and deliver it to the police laboratory for analysis. The laboratory finds the defendant’s fingerprints on the knife. At trial, it will take several witnesses to establish that between the defendant’s hand and the laboratory technician’s analysis the knife was not altered: the victim will testify that s/he saw the knife in the defendant’s hand, the police officer at the scene will testify that s/he possessed the knife and transported it from the scene to the laboratory without losing sight of it, and the laboratory technician will testify to receiving it from the same police officer. This establishes a chain of custody for the knife - the knife was accounted for the entire time and has not been altered.