Mexico City ? Here’s an interesting detail about the much-publicized recent arrest of Mexico’s top drug baron Edgar Valdez Villarreal, better known as “La Barbie” — he was caught with a U.S.-made M-16 semiautomatic rifle and other sophisticated arms that Mexican officials suspect were smuggled from the United States.

In Mexico, U.S. arms smuggling is a big issue. President Felipe Calderon said during a visit to Washington in May that of all the guns and assault rifles seized in Mexico over the last three years, “more than 80 percent of those we have been able to trace came from the United States.”

Mexico’s drug cartels have become increasingly well-equipped armies thanks to a flood of semi-automatic U.S. weapons, which have been easier to get since the U.S. government allowed a 10-year ban on sales of assault weapons to expire in 2004, Mexican officials say.

Renewed sales of U.S. semiautomatic weapons and Calderon’s military offensive against the drug cartels help explain an escalation of drug-related violence that has left more than 28,000 dead over the past four years, they say.

Does Mexico have a point in blaming Washington for part of its bloodshed?

Or is it conveniently passing on the blame to the United States for its own drug violence?

According to a recently released U.N. report titled “The Globalization of Crime,” most of the firearms used by Mexico’s drug cartels are purchased in the estimated 6,700 gun shops along the U.S. border with Mexico.

Smugglers use “straw purchasers” to buy the weapons, and then take them across the border concealed in some of the 88 million private cars that make border crossings each year. An estimated 20,000 mostly sophisticated weapons are trafficked annually south of the U.S. border, it said.

A U.S. Government Accountability Office report last year concluded that “available evidence indicates that a large proportion of firearms fueling Mexican drug violence originated in the United States, including a growing number of increasingly lethal weapons.”

Mexican officials say the problem has worsened since former President George W. Bush allowed the 10-year ban on sales of assault weapons to expire. Mexican drug cartels have found it much easier to buy semi-automatic rifles in U.S. gun shops because these weapons can be easily resold without leaving any traces, they say.

The National Rifle Association, the biggest U.S. gun lobby, is skeptical about Mexico’s claims that most of the drug cartels’ weapons are smuggled from the United States.

Many of the tens of thousands of Mexican army troops who have defected to the drug cartels in recent years have taken their heavy weapons with them, NRA officials say.

So who is to blame, I asked University of Miami professor Bruce Bagley, who has written several books on Mexico’s law enforcement problems.

“The U.S. has a responsibility, and we had not fulfilled it until (Homeland Security Secretary) Janet Napolitano began to tighten up the U.S. side of the border,” Bagley said. “But the Mexicans have to assume responsibility for the security of their own borders, and they have not done so.”

Mexico’s customs and border patrol units are corrupt, and most of the purges designed to modernize them have been only cosmetic, Bagley said. Mexico should invest more money into improving its law enforcement agencies, he added.

My opinion: I agree. Mexico has the primary responsibility for ending its drug violence. Unless it becomes serious about reforming its 2,200 — yes, you read right — corruption-ridden police forces to prevent them from protecting drug traffickers, Mexico’s drug cartels will always be ahead of the game.

But President Barack Obama could help reduce Mexico’s bloodshed if he restored the expired U.S. ban on sales of assault weapons, as he promised to do during the campaign, or if he signed an existing inter-American drug convention known as CIFTA, which requires signatory countries to better track the end users of firearms sales.

The Obama administration has not done either. Meanwhile, as the recent capture of “La Barbie” shows, Mexico’s drug barons continue to get increasingly more — and more lethal — U.S.-made weapons.