I have spoken to ATF personnel who have been in Mexico and have worked the border area dealing with the flood of illegal arms into Mexico. Each one said the arms they are seeing are being manufactured overseas.
Their reason behind this statement is that there are no serial numbers on the firearms. Depending on what country, there are laws that govern the manufacture of arms, so the serial number may be on the barrell, slide, frame, or all parts. These weapons don't have any numbers, not ground off, just not ever serialized. Why don't we hear about that on the news?!?