Fox News tends to be rather strident, but using a high-school level of rigor in their research*, they find that about 83% of weapons seized in Mexico do not originate in the U.S.
Here's why: of the weapons seized by Mexican authorities, the information of only about 1/3 of the weapons seized are submitted to the U.S. for tracing. ALL U.S. firearms are clearly marked and have a serial number. If there are no markings, there is no point submitting the weapon for tracing, since it obviously did not come from the U.S. Of the information submitted to the U.S. less than half could be traced. Of the ones successfully traced, 90% came from the U.S.- but the total amount of guns seized traced to the U.S. is only about 17% of the total amount of weapons.
*meaning U.S. political mouthpieces either can't do 8th-grader research or are deliberately lying.