Who is usually first to arrive at a crime scene and works to secure the scene so it cannot be contaminated?

Police officer, also known as the first responder

Explanation:

A crime scene in an investigation may be defined as a location or an area when a possible crime had taken place. It is the area of interest of the crime investigating team to investigate and analyze the crime scene to find any evidence against the crime.

The police officer is the first person who arrives at the crime scene and his first and main objective is to protect the crime scene so that the area is not contaminated and the evidences are not lost. He is also known as the first responder. Crime scenes contain the physical evidences that can be pertinent to any criminal investigation