Mexico has long complained that drug gangs are terrorizing cities with high-powered weapons smuggled from the United States. But Mexican lawmakers are now up in arms over the recent revelation that the US Bureau of Alcohol, Tobacco, Firearms and Explosives (ATF) purposefully allows some of these weapons to be smuggled south of the border so it can track them as part of "Operation Fast and Furious."

“[The operation] is a grave violation of international rights,” Jorge Carlos Ramírez Marín, president of Mexico's lower house of Congress, said Tuesday. “What will happen if next time they’ll need to funnel in trained assassins, for example, or nuclear arms?”

Fellow congressman Humberto Trevino claims that an estimated 150 shooting injuries or deaths have been linked to guns that were allowed by US agents to proceed into Mexico.

IN PICTURES: Mexico's drug war

The legislators are calling for a joint US-Mexico working group to examine Operation Fast and Furious. Some also proposed sending a congressional delegation to Washington to press for more action against gun trafficking, which plays a crucial role in fueling a drug war that has killed 35,000 people here since December 2006.

Mexico’s Foreign Ministry on Saturday requested “detailed information” from US authorities about the operation.

How 'Fast and Furious' backfired

A Phoenix-based operation, Fast and Furious backfired when two of the weapons in the program were found at the scene of a border shootout that killed US Border Patrol agent Brian Terry in December. ATF whistleblowers and some US congressmen kicked up a fuss over a program they feared would aid cartels in killing innocent people. And last month, a gun smuggled from the US was used in the killing of US Special Agent Jamie Zapata outside Mexico City, although it has not been determined that the gun was part of Fast and Furious.

As a result, last week the ATF announced a review of its firearms trafficking strategies.

The controversy comes just as US-Mexico relations appeared to be on the mend following last week’s visit by President Felipe Calderón to the White House. But this latest problem seems to show that not much of substance was resolved at the leaders meeting, some experts say.

Javier Oliva, a security expert at Mexico’s National Autonomous University, wonders whether Mexico even knew about the operation. “The fact that the Mexican government is requesting information demonstrates that there isn’t much collaboration with the United States,” he says.

Mexico will use the ATF controversy to keep pressuring the US on arms trafficking, says Vanda Felbab-Brown, a Mexico expert at the Brookings Institution in Washington.

“Now [Mexico] can say that the US is equally lacking in intelligence, and are equally having problems with efficiency of [its] programs,” she says. “It allows the Mexican government to deflect the focus from problems in Mexican institutions.”

1,200 guns on the loose?

According to the Center for Public Integrity in Washington, a public-interest investigative organization that has done extensive reporting on the case, only 10 percent of the 2,000 guns that Fast and Furious allowed to gunrunners to purchase were eventually recovered in Mexico. Close to 30 percent – or 600 guns – were recovered in the US. The remaining 1,200 guns have not been recovered and possibly remain in the hands of drug gangs.

But law enforcement officials and some experts, including Ms. Felbab-Brown, argue that such sting operations lead to the capture of high level traffickers, and not mere straw purchasers who can be easily replaced.

The ATF's current investigation into the operation is essential, however, adds Felbab-Brown, to determine whether the tracked guns that wound up in shootouts were a result of corruption on either side of the border, or due to error.

IN PICTURES: Mexico's drug war