In a major front-page story today, The Washington Post exposes a dirty secret that complicates the U.S.-Mexico cooperation against drug cartlets. The Post's Manuel Roig-Franzia reports that U.S. guns are arming Mexican drug cartel killings.

The U.S. weapons -- as many as 2,000 enter Mexico each day, according to a Mexican government study -- are crucial tools in an astoundingly barbaric war between rival cartels that has cost 4,000 lives in the past 18 months and sent law enforcement agencies in Washington and Mexico City into crisis mode.

And at least two states are directly implicated; a fact that could be met with calls for emergency reforms from some officials:

Arizona and Texas have become a "gunrunner's paradise," according to Garen Wintemute, a professor at the University of California at Davis who published a study on gun buying in the Southwest. Licensed dealers must conduct background checks, but unlicensed sellers can sell "personal collections" at weekend gun shows without background checks.

We've been blogging about newly announced strategic partnership between the two countries and proposed U.S.-Mexico anti-drug efforts.

What do you think can be done to cut off the flow of American guns to Mexican narcotics trafficers?