At last rough check Ron, I believe the combined IAAI NAFI numbers put the total at about 12000. It is going to be very hard to catergorize the true number of FI because under 41/50 states with PI laws, ANY PI can legally conduct FI's. While many PIs don't, you would be amazed to find out just how many non-certified non-formally trained PIs are doing the work as Fire Investigators for arson/arson defense cases. Also, you would have to then add in a percentage number of engineers in each state also because they are conduct "investigations" under their license.

I hope I didn't ruin your day, but like I said before, the PI licensure issues vary state to state in their applicability, but most states have the following definition in their PI Law Statutes:

“(2) “Investigator” means a person who is a licensed investigator under (State Statute Citation) and who engages in the business of obtaining or furnishing, or who solicits or accepts employment to obtain or furnish, information about:
(a) Crimes or wrongs done or threatened against the United States or any state or territory of the United States;
(b) The identity, habits, conduct, business, occupation, honesty, integrity, credibility, knowledge, trustworthiness, efficiency, loyalty, activities, movements, whereabouts, affiliations, associations, transactions, acts, reputation or character of any person;
(c) The location, disposition or recovery of lost or stolen property;
(d) The cause of or responsibility for fires, libels, losses, accidents, damages or injuries to persons or property; or
(e) Evidence to be used before any court, board, officer, referee, arbitrator or investigation committee.”