It is usually a call to the emergency services that triggers the investigation of a potential crime. That is why the first person on the scene, known as the first responder, is usually a police, fire, or medical officer. His or her priority is always the safety of those who are at the scene, but the responder also has to be aware of the importance of preserving evidence that may be relevant to any crime that has been committed. After all, he or she is the only person to see the location in its original state. The actions and observations of first responders can therefore be crucial in terms of gathering and preservation of evidence that may eventually be presented in court.
On arrival at the scene, the first responder will carry out an initial assessment of whether a crime has actually been committed. If there is an obvious victim, the first priority has to be to offer first aid and any other assistance. In the case of a serious crime, the first responder will call for help so that the tasks of dealing with those present and preserving evidence can be delegated.
Although the first responder's first priority is assistance rather than looking for evidence, he or she will still keep the latter in mind in all their actions. However, the destruction of evidence is acceptable if it is needed to help a victim or even save their life. Whatever first responders do has the potential to affect evidence, and they need to be aware of this. Their first task is to assist any victims present at the scene of crime. They will also look for any suspects and arrest them if possible. Witnesses must be detained and kept separately. This is to stop them sharing from their stories and contaminating evidence. Should the first responder see suspects, victims, or witnesses trying to clean up or dispose of evidence, then they have a duty to stop them in the interest of preserving the scene.
The first responder will then secure the scene of crime by taping it off and taking careful note of who comes in and out. Entry is restricted to those who have a need to be there. Evidence outside this barrier may be easily contaminated, so the first responder should make the sealed-off area as wide as is practicably possible. The cordon should be policed and arrangements made to deal with the public and press, giving them information without allowing access that could otherwise disturb evidence.
When the first responder attends to a victim, he or she may need to move the person or any evidence. When this happens, a record should be taken of any actions that are performed. The original position and posture of the body and anything that is moved should be carefully recorded. The victim could need to go to the hospital and, ideally, someone would go with him or her. However, should the first responder be alone, the duty is to remain at the scene.
In the case of a serious crime, forensic investigators will be called to the scene. It is important for the first responder to mark out a common approach path for those coming to the scene later. This reduces the possibility of disturbing or contaminating the evidence at the scene, which can easily happen if people are moving at random through the site. Often the common approach path is from the cordon to the focal point of a crime, such as a body. Possible entry and exit points of perpetrators will also be avoided so that valuable evidence is not disturbed.
First responders must always minimize the impact they themselves have on the scene. They should attempt to minimize their own fingerprints or other trace left evidence behind and take special care to avoid areas and items that may contain suspects' fingerprints, such as doorknobs and light switches. Everything they do should be carefully recorded in case it alters the original crime scene. Sooner or later the first responder will be joined or replaced by others. It is particularly important that a record of what has been observed and carried out by the first responder is handed over by those who are next on the scene of crime. The chain of custody of evidence begins with first responders, even though they are not forensic experts. Their actions at the scene can be vital in the preservation of important evidence.
see also Crime scene investigation.