In a radio interview, Colom said a Los Zetas hit team that stormed a ranch in the Peten and beheaded laborers, three of them minors, may have included former commandos from a special forces unit of Guatemala's army, the Kaibiles.

"It was total savagery. Everyone is in a stupor," Colom told Mexico's MVS radio network, adding that a feud between rival drug gangs led to the massacre.

Guatemala's once-fearsome army is a shadow of what it was, and doubts arose over whether it could restore order in the sprawling Peten — the northern third of the nation — where Colom declared a 30-day state of siege Monday night. The region is part of a crucial cocaine-smuggling corridor to the United States.

Since a 36-year civil war came to an end in 1996, army strength has fallen from 50,000 troops to about 16,100 today, marking the profound transformation of a nation once dominated by military repression but also leaving areas such as the Peten wide open for drug traffickers.

Some 40 to 50 assailants under a Mexican commander arrived on Los Cocos ranch Saturday night looking for ranch owner Otto Salguero, systematically killing and beheading unarmed laborers, authorities said. By Sunday morning, 25 workers had been beheaded and two others also were killed. Some of the laborers had been hired in another region of Guatemala only a week earlier and had come to the ranch for temporary work.

Colom said three survivors were under government protection, including a pregnant woman whose pleas for mercy were heeded even as killers beheaded her husband.

"Obviously, the woman is in a state of shock," Colom said. "We hope that when her emotional state normalizes we'll have a few more details."

Photos in Guatemalan newspapers showed that the killers used the blood of victims to write threats on ranch walls, signing off as "Z 200," a branch of the Zetas. Colom said that some of the beheaded corpses were disfigured with Zeta markings.

Colom said that Salguero, whose whereabouts were unknown, had four ranches in the area. The Z 200 group was thought to be responsible for killing Salguero's niece Friday near the town of Sayaxche in the Peten, where she arrived to pay a ransom for her kidnapped husband. Her murderers cut her body into four pieces. Her husband remains missing.

Los Zetas have operated hand in hand with Guatemalan drug clans, but they've attempted to crush one group, the Leon family. Its commandos killed Juan Jose "Juancho" Leon in 2008, and a brother who took over the business, Haroldo, was slain in the Peten on Friday.

Whether Salguero, the ranch owner, had relations with the Leon clan wasn't known.

Colom cast the weekend massacre as "a crime against humanity," raising the prospect that Guatemala may seek more direct international help to battle the onslaught of organized crime.

Los Zetas arose in the late 1990s as enforcers for the Gulf Cartel in Mexico's northeast Tamaulipas state. Some of its leaders are deserters from an elite Mexican army unit. A little more than a year ago, the group broke away to form its own drug-trafficking group.

Zeta tentacles now stretch across Central America and into the Andes, source of the world's cocaine. The group has recruited former Guatemalan Kaibiles commandos to help with territorial control in the Peten, counter-drug officials say.

The group has made brutality a hallmark, using beheadings and what some term "hyper-violence" as a tactic to frighten competitors and hollow out regions where it seeks uninterrupted control. Los Zetas are blamed for killing 72 migrants in northeast Mexico in August.

Colom said a former Guatemalan army officer was among three people arrested Monday with alleged links to the weekend massacre. He didn't say whether the man was a former Kaibil.

It marked the second time that Colom had declared a state of siege in recent months. In December, he placed Alta Verapaz province, in north-central Guatemala just south of the Peten, considered the cradle of Zetas operations in Guatemala, under a two-month siege. The Zetas are thought to have returned there once the siege ended.

A state of siege suspends freedoms of assembly, movement and expression and the right to bear arms, and gives overall control of the Peten to the army.