We've seen quotes implying that the ATF has confirmed 90% of recovered Mexican crime guns come from US sources. This is a perfect example of the "whisper-down-the-line" numbers used by anti-gun groups.
What was really said was that 90% of guns submitted for tracing were traced to a US source. What is ignored is that there are a lot of guns that obviously do not come from the US--These guns were not submitted. Guns made in or imported to the US have to have manufacturers or importers marks, making it fairly easy to determine that many guns do NOT have a US source.
If you include the total number of Mexican recovered firearms, about 17% were traced to the US. Honestly, that number surprises me, I would not have thought it would be that high.