Made in Romania, reconfigured in Vermont with more lethal features, and marketed along the Southwest border, the WASR-10 has become the #1 gun recovered at Mexican crime scenes.

A semi-automatic variant of the AK-47, the WASR-10 is one of the most popular "weapons of choice" among Mexico's drug cartels and a central player in the ongoing drug war that has claimed almost 35,000 lives over the past four years.

This Web-exclusive investigation of the Romanian AK tracks the circuitous route it travels, extending more than 7,000 miles from Romania through the U.S. and into Mexico. Along the way, the gun is transformed from a slimmed-down "sporting purposes" version -- to pass the import test -- into a lethal assault weapon that is heavily marketed in Southwest gun stores.

The journey of the WASR-10 sheds light on how firearms' importers circumnavigate an import ban that's been on the books for more than 40 years. And it reveals the limitations of law-enforcement efforts to staunch the flow of weapons into Mexico.

A joint investigation by FRONTLINE, the Investigative Reporting Workshop, The Center for Public Integrity, InSight and the Romanian Centre for Investigative Journalism.