Editor's note: This article is part of "Underworld: a global crime blotter," a semi-regular series covering crime and punishment around the world.

IXTEPEC, Mexico — The mass kidnapping in this southern Mexican state of Oaxaca followed a modus operandi that has become depressingly familiar.

Gangsters in January held up a freight train on which Central American migrants were traveling north to the United States. More than 50 victims were marched off at gunpoint to be held in a safe house until relatives coughed up ransom money.

But whereas migrant abductions normally don’t lead to arrests, rising international pressure led to Mexico’s elite marines searching for the Oaxaca kidnappers.

The roundup of 10 suspects revealed a dangerous development — the alleged kidnappers included both Mexico’s Zetas and Mara Salvatrucha gangbangers from Guatemala, El Salvador and Honduras.

It was evidence of a strengthening alliance between Mexico’s criminal army led by former special forces commandos and a web of street gangs that stretches from Los Angeles through the whole of Central America.

Reports of the Zetas and Maras doing drug deals together or assassinating mutual enemies have been floating around for several years.

But human rights workers and police in southern Mexico and Guatemala say they have now formed a more concrete alliance, in which they work together on kidnappings and acts of intimidation and terror.

Alberto Donis, who works at a migrant shelter in Ixtepec, said he recently received threats from a cell of Maras who work alongside the Zetas in kidnapping.

The threats came after the Catholic-run shelter had exposed the mass abductions of migrants in the media, creating the pressure that led to the intervention of the marines.

“We know that we affect some big business interests and that puts us in danger. We are a stone in their path,” said Donis, who is originally from Guatemala.

Donis said the Maras often work as spotters for the kidnapping gangs, moving on the freight trains and in the migrant shelters to identify which travelers look the easiest targets and have families already in the United States. Relatives of the kidnapped migrants are normally forced to pay $1,000 to $5,000 for their release.

On other occasions, Donis said, Maras work as gunmen alongside Zeta commandos in the kidnappings.

“In some ways, the Maras have become like employees of the Zetas,” said Donis. “They are people who have had lack of opportunities that have led them into this violent life, in which they prey on their own countrymen.”

Originally founded as a street gang in Los Angeles, the Maras have also moved into rackets such as narcotics and extortion. Likewise, the Zetas, who began as enforcers for the Gulf drug trafficking cartel, have diversified into a number of criminal enterprises.

Their deadly alliance is also strengthening over Mexico’s southern border.

In December, the Guatemalan army declared martial law in its northern jungles in reaction to a growth of Zeta commandos and training camps.

One of the reasons for the action was that the Zetas were allegedly giving paramilitary training to Mara gang members in the area, heightening the violence on Guatemala’s already bloody streets.

In response to the martial law, alleged Zetas forced a Guatemalan radio station to broadcast a threatening statement.

“We will start the war in this country, in malls, in schools and in police stations,” said the message. It added, “If this message is not put on the air in an hour, the radio station will burn … . The families of those who work at the station will be executed if you do not read it.”

In a possible follow-up to that threat, assassins set off a bomb on a bus in Guatemala City killing six people in January.

Guatemalan police alleged that members from the Mara 18 gang were behind the blast, but said it could have been on orders from the Zetas.

The explosive was set off from a cell phone — making it a remote operating device, of a similar complexity to those used in Iraq and Afghanistan.

Another bomb was set off by a similar device in the Mexican state of Hidalgo in January, killing a policeman and injuring several others.

Near that bomb was a note signed with the letter “Z.” – a common shorthand for the Zetas.

“This is the beginning, and a little sign of what it means to those who want war,” it said in the note.

The hellish alliance could also be reaching deeper into Central America.

El Salvador’s newspaper El Diario de Hoy reported that on Jan. 1, chiefs of the Mara 18 gang met with Zetas in the Salvadoran town of Llopango.

In the meeting, the Zetas reportedly offered for the gangbangers to send their hardest veterans for paramilitary training.

For human rights workers such as Donis, confronting these gangs is extremely dangerous.

But Donis said the threats will not stop him fighting for poor migrants.

“If we don’t fight for these migrants, then who will? The authorities. We are doing the job that the authorities should be doing,” Donis said. “We know that it is risky, but the love from God is strong. We trust that God is with is.”