Just as they have done in the north, groups claiming to be Zetas have set up criminal networks to control transit routes for drugs, migrants and contraband such as pirated DVDS, intimidating the populace and committing gruesome murders as an example to the uncooperative.

Four years ago they started preying on the south, Mexico's poorest region. They moved into Oaxaca, Chiapas and other southern states and then northern Guatemala, where attacks on townspeople became so commonplace that the government last month sent in 300 troops to regain control of the border province of Alta Verapaz.

In towns on the Oaxacan isthmus and the center of Oaxaca city, the capital, the wealthy as well as street vendors and migrants have been kidnapped and subjected to extortion.

Then last month, the gang blamed for massacring 72 migrants in the summer in the northern state of Tamaulipas became suspects in the disappearance of more than 40 Central American migrants in Oaxaca. The abduction drew international attention when the El Salvadoran foreign ministry reported the crime, but the Mexican government initially denied it happened.

The travelers were last seen Dec. 16 near the city of Ixtepec, along the sun-scorched transit route for thousands riding northbound freight trains. Twenty escaped and took refuge at a migrant shelter run by the Rev. Alejandro Solalinde, who says he has learned the kidnappers have ties to the Zetas.

The Mexican Attorney General's Office announced the arrest this month of a Nicaraguan and a Mexican on suspicion of being involved, but said nothing about Zetas or the missing migrants.

The Mexicans say the Zetas have hired Guatemalan former counterinsurgency soldiers to train new recruits, and a Zetas training camp for hit men was uncovered on the Guatemalan border last year.

Alejandro Poire, Mexico's government spokesman for security issues, said the reported scope of Zetas activity in southern Mexico is hardly comparable to the turf battle raging between the Zetas and their competitors in the north, where a split from their former employers, the Gulf Cartel, has sparked regular grenade attacks and daylight shootouts.

But to Solalinde, the Zetas "are a terrible de facto power."

"Unfortunately, we have a very corrupt country, with law enforcement agencies infiltrated" by organized crime, the priest said.

Four days after Solalinde reported the kidnapping and named the Zetas, he was visited by a burly, shaven-headed man whom police identified as a known hit man.

Police now patrol outside the shelter of unfinished cinderblock rooms, where migrants sleep on cardboard or blankets and stray dogs and cats wander about.

"There is danger," Solalinde said. "But imagine if every single person in Mexico kept silent, if all looked the other way, if no one did anything? That would be terrible for Mexico."

The Zetas rule by fear, threatening police, city officials, journalists and anyone else who gets in their way.

In November, on a much-visited cliff overlooking the picturesque center of Oaxaca City, police found a severed head in a gift-wrapped box. A threatening message left with the head was signed "Z," apparently for Zetas.

In the Oaxacan city of Juchitan, a decapitated man was dumped by a road in November and another was found chopped up in May with a note saying he was killed for posing as a Zeta.

"There are places, cantinas, where we all know they sell drugs, where the Zetas get together. Everybody knows, but nobody does anything," said a local journalist who requested anonymity fearing reprisals.

Authorities, however, contest the notion they are doing nothing. In Chiapas state, on the Guatemala border, more than 240 local and state police officers have been fired or arrested since 2008 for having links to the Zetas, according to the state Public Safety Department.

The Zetas formed in the late 1990s from a small group of elite soldiers based in Tamaulipas who deserted to work for the Gulf drug cartel.

They earned their notoriety by becoming the first to publicly display their beheaded rivals, most infamously two police officers in April 2006 in the Pacific resort city of Acapulco. The severed heads were found on spikes outside a government building with a message signed "Z'' that said: "So that you learn to respect."

That year, the Gulf cartel, emboldened after retaining control of the northern border city of Nuevo Laredo, sent the Zetas to take over the south, which they kept after their boss, Gulf leader Osiel Cardenas Guillen, was extradited to the U.S.

By 2008, the Zetas had operations in 28 major Mexican cities, according to an analysis by Grupo Savant, a Washington-based security think tank.

They operate unchallenged in the south, the think tank says. While other cartels are preoccupied with maintaining their Pacific coast ports and northern border transit routes, the Zetas make hundreds of millions of dollars from extortion and trafficked goods coming overland via Guatemala.

Mexico's federal government acknowledges that Zetas have no geographic concentration like other cartels and therefore have shown up in disparate parts of the country. They operate almost like franchises, sending one member to an area they want to control to recruit local criminals.

For Central Americans migrating north, there are few options but to risk their lives crossing Zetas-controlled territory.

At the migrant shelter in Ixtepec, Denis Torres, a 24-year-old bricklayer from Tegucigalpa, Honduras, said he set out on his journey despite his family's pleas to stay. He said he was determined to join his uncle in Miami, where he had been promised a construction job.

"You do travel in fear, thinking they can kidnap you and torture you or kill you just because you came pursuing the American dream," he said.

**Associated Press writers Juan Carlos Llorca in Guatemala City; Manuel de la Cruz in Tuxtla Gutierrez, Chiapas; and Sayra Cruz in Oaxaca City contributed to this report.