Reduction In USA's Violent Crime. Why?

It has been reported that the rates of violent crime in the USA have been steadily going down over the last few years.

WHY?

Some have proposed that that decrease is parallel-to, if not caused-by, the increase in the number of
law-abiding (And legally presumed sane) citizens who possess modern and effective (ie At police or military grade, exempting fully automatic) guns.(Yes, I know that "coincidence is not causation").

I think that proposition is worthy of intensive study as would require the consideration of the following variables:
1. The races of violent criminals and their victims;
2. The number of all sworn peace officers, per square mile, in each jurisdiction;
3. The rated laws regarding the purchase and carrying (Concealed) firearms;
4. The decrease or increase rates of violent crimes in each jurisdiction;
5. The number of firearms seized in each jurisdiction and the portion of those seizures as resulted in criminal convictions;
6. The portion of each place's population as are illegal immigrants; And,
7. Other factors as fair-minded and informed researchers might examine.