President Obama used a speech at Mexico’s Museo Nacional de Antropología – the National Anthropology Museum – to claim that ‘most of the guns used to commit violence here in Mexico come from the United States.’
That concept has been debunked many times over - here's a analysis of the numbers from Stratfor from 2011, and many more are available.
This a a recycling of the old claim that "over 90 percent of the firearms originate from the U.S." when discussing seized weapons in Mexico in 2008. What is left out, though, is that it's 90 percent of the traced guns - but a large percentage of the guns recovered in Mexico do not get sent back to the U.S. for tracing, because it is obvious from their markings that they do not come from the U.S. The actual number was 3,480 out of 30K, or about 12%.