When did the "90 percent lie" unravel? At worst it seems to be an unsupported exaggeration. If what Coulter says is correct, then all that we can say is that we don't know what percentage of illegal guns in Mexico come from the U.S.
One thing that we can say is that of Mexico's neighbors the U.S. is the easiest place to purchase firearms and that Arizona is the easiest place in the U.S. to do it. Also, since the cartels have huge amounts of cash stuck in the U.S., buying weapons and smuggling them in is a way to get rid of a small part of that hoard.
So, if we do not know the actual number (and firm numbers are always suspicious no matter who gives them) logic would seem to indicate that the preponderance of them come from the U.S. i