According to a report by newswire EFE, Castañada did not specify when or where these robberies took place. Presumably among the cases he was referring to was one documented in March 2009, when authorities found weapons at a Zetas training camp in the northern Quiche department. The stash was later traced to a military warehouse in the capital.

When the police raided another alleged Zetas warehouse outside the capital in April 2009, they found more weapons from the same source. These included nearly 600 grenades, eleven machine guns and 5,000 rounds of ammunition, stored in boxes labeled "Guatemalan Military Industry." Police also found landmines and military uniforms during the raid.

The Zetas are thought to have obtained these weapons from the Mariscal Zavala military base, in sales (or theft) that took place between July 2007 and January 2008.

On August 20, 2009, the Zetas reportedly stole another arms shipment, including rocket and grenade launchers, that were apparently traveling overland in Guatemala, reports La Jornada.

Guatemala's weapons black market is mostly controlled by shadowy criminal organizations evolved from the Cold War-era security forces. These include active and retired military and government officials, who worked together in counter-insurgency, intelligence, special forces, and the death squads. Sometimes compared to a clandestine security apparatus or a parallel state, these operatives are described as the "hidden forces" running the country's criminal networks: extortion, kidnapping, money laundering, smuggling migrants, drug trafficking, and the illicit arms trade.

The political and military ties of the Illegal Corps and Clandestine Security Apparatus (Cuerpos Ilegales y Aparatos Clandestinos de Seguridad - CIACS), as the group is known, differentiates the organization from traditional smugglers like Juan Chamale or the Lorenzanas, as well as street gangs MS-13 and Barrio 18.

The connections between the CIACS and Mexican gangs like the Zetas are well known. Rogue members of the Kaibiles, or Guatemalan Special Forces, can earn up to $5,000 a month training the Zetas in combat, the Guatemalan security minister said this week in Cancun.

The question of how far Guatemalan arsenals are arming Mexican groups may have indirect implications for U.S. policy regarding gun trafficking to Mexico. When the U.S. Government Accountability Office reported in June 2009 that over 90 percent of the arms seized in Mexico (and that authorities were able to trace) were traced to the United States, debate flared on both sides of the border. Mexico pointed to U.S. complicity in the so-called "drugs war," with gun stores in border states arming gangs like the Zetas, as documented in InSight's GunRunners project.

The argument that Mexican groups are instead obtaining Guatemalan weapons could prove attractive for U.S. political interests seeking to justify lax gun laws. But identifying the greater source of arms for Mexican cartels -- Guatemala or the U.S. -- means relying on estimates and statistics that still seem fuzzy.

As reported by the AFP, one top U.S. military official recently suggested that Mexican groups are arming themselves mostly from Central American arsenals. During testimony before the U.S. Senate on March 30, General Douglas Fraser, head of the U.S. Southern Command, said, "Over 50 percent of the military-type weapons that are flowing throughout the region have a large source between Central American stockpiles, if you will, left over from wars and conflicts in the past."

This seems to echo statements reportedly made in a U.S. Diplomatic cable which Mexican news daily La Jornada said it exclusively obtained from whistleblower site WikiLeaks. According to the newspaper, the cable, reference ID 09Mexico808, was drafted as a briefing for ATF agents following an arms trafficking conference on April 1-2, 2009. The cable says that 90 percent of high-power weapons submitted by Mexico law enforcement to the U.S. for tracing were in fact linked to Central American arsenals, the newspaper reports. La Jornada has not yet made this cable available.

Mexican groups have doubtlessly obtained AK-47s and grenades from Cold War arsenals in Guatemala, and even El Salvador. But breaking down the origins of Mexico's arms supply, in order to assign political responsibility for the gun trafficking problem, will likely prove fruitless. What is clear is that the U.S. has a set of laws that are indirectly feeding weapons into the hands of Mexican gangs, while in Guatemala the problem is more akin to a lack of proper law enforcement.

Addressing the weapons trade means confronting impunity within Guatemala's police and army. But that country's Congress and executive branch have often proved powerless to take on the shadowy forces still controlling the region's illicit arms networks.

One indication: the Arms and Munitions Law, which took ten years to draft and which was finally implemented in 2009, reduced the number of guns a person could carry from twelve to three. "The law seems to have been designed to aid the dirty business of arms trafficking," observed a member of Congress in a December 2010, as quoted in a report by the Woodrow Wilson Center.

It is clear that both Guatemala and the U.S. are prime sources of weapons for Mexican gangs. Measuring how much seems less important than addressing the laws in both countries that aid the "dirty business."