Once it has been determined or believed that a crime has been committed, the chain of custody of evidence must be adhered to in order for some items to be admitted into court as part of a trial. Any item can become evidence, for example: a car, a house, a yard, a piece of clothing, a knife, a gun, your body, visible injuries, etc. The best advise is "DO NOT TOUCH, MOVE, OR WASH ANYTHING." If you are in doubt, wait for law enforcement instructions on what to do.