According to the Washington Post, they come from the United States:
No other state has produced more guns seized by police in the brutal Mexican drug wars than Texas. In the Lone Star State, no other city has more guns linked to Mexican crime scenes than Houston.
Of course, that is a bit of a misdirection, as others have shown. However, why isn’t anyone talking about taking the guns away from the US Border Patrol, in light of this story?
A Mexican woman is under arrest after agents at the Andrade Port of Entry discover a stolen gun in her possession.The 25-year old woman was taken into custody on Saturday after agents at the border discovered the woman had a gun they say belongs to the US Border Patrol.