On their third visit, they found a firing squad.
Gunmen unleashed a furious barrage of bullets and at least one grenade, in some cases finishing the job point-blank. When the shooting stopped that day in April, five of the 10 Guatemalan agents lay dead and a sixth was wounded.
The fleeing killers, identified by authorities as members of the Mexican drug gang known as the Zetas, left behind a cargo truck packed with 700 pounds of cocaine. More stunning was the cache found in a brick warehouse: 11 M-60 machine guns, eight Claymore mines, a Chinese-made antitank rocket, more than 500 grenades, commando uniforms, bulletproof vests and thousands of rounds of ammunition.
"They were preparing for war," said the adjunct director of the National Civilian Police, Rember Larios.
As Mexican President Felipe Calderon presses a 2 1/2 -year-old offensive against narcotics traffickers in his country, the war has spilled south into Guatemala, where proximity, weak law enforcement and deeply rooted corruption provide fertile ground for Mexico's gangs, say officials and analysts in the region.
During the last year and a half, the Zetas have carved a bloody trail across Guatemala's northern and eastern provinces. More than 6,000 people were slain in Guatemala in 2008. Police say most of the killings were linked to the drug trade.
As the recent blood bath shows, the violence is now threatening the capital, deep in the interior.
Authorities say Mexican drug gangs, primarily the Zetas and rivals from the state of Sinaloa, are ramping up operations in Central America to evade increased marine patrols near Mexico as they relay drug shipments to the United States and Europe.
The gangs are also ferrying military-style weapons north into Mexico to fight Calderon's forces and opposing gangsters while also vying to take over street sales in Guatemala. Some of the weapons are left over from the wars that the United States helped fight in Central America -- including here in Guatemala, which is still recovering from its 36-year civil war.
"They're looking for new areas," said the U.S. official, who spoke on condition of anonymity because he wasn't authorized to comment on the matter. "They need a place where they can operate with impunity."
Since January 2008, Guatemalan police have arrested about 30 suspects who they said were working for the Zetas, the armed wing of the so-called Gulf cartel, based in northeastern Mexico.
The Zetas, formed in the 1990s from former Mexican special forces, have shaken Mexico in recent years through hundreds of well-planned killings and an expanding reach. The group has decapitated numerous rivals, dumping the heads in public places with menacing messages.
Authorities on edge
The spreading influence of Mexican traffickers has Guatemalan authorities on edge and is beginning to stir concern in Washington that powerful drug gangs could imperil fragile Guatemala and its weak neighbor, Honduras.
U.S. Rep. Eliot L. Engel (D-N.Y.) urged Secretary of State Hillary Rodham Clinton last month to steer more law-enforcement help to Guatemala, warning that it is even weaker than Mexico.
"It is essential that we view our efforts to combat drugs and violence in the Western Hemisphere in a more holistic way," said Engel, who chairs the Western Hemisphere subcommittee of the House Committee on Foreign Affairs.
Guatemalan police commanders say their 20,000 officers cannot match the firepower of the Mexican traffickers, who have made growing use in Mexico of military-type arms, such as 40-millimeter grenades and .50-caliber rifles capable of piercing armor.
Recent seizures in Guatemala have yielded similar weapons. "These are things we have seen only in photos of Iraq and the Gulf," said Larios, the police commander. "Not in Guatemala."