Investigation and expertise
These are, like the accidental witness, though not in chief of accidental access to relevant information but through their skills, experience and equipment, used to provide information to the actual decision makers above
coroners, medical examiners, and other medical experts
other judicial experts in various fields, such as state certified appraisers, certified public accountants and other professionally licensed or certified persons retained by the parties to give expert advice, the testimony and exhibits of which is admitted by the Court.
amicus curiae is a vague term for other persons consulted by the court
marshal, sheriff, constable