Mara Salvatrucha, or MS-13, became El Salvador's deadliest gang through force of numbers and the power of the handgun – while inking some pretty crazy tattoos. Now if they weren't deadly enough, the gang is transitioning into adopting heavier weapons while teaming up with Mexico's Zetas.

It's not easy tracking guns inside the gangster's paradise of El Salvador, as the Maras are not prone to advertising their alliances and hardware. But according to a recent report (.pdf) from Douglas Farah and Pamela Phillips Lum of the International Assessment and Strategy Center, a hawkish national security think-tank, the gang is moving "away from a dependence on handguns via the acquisition of automatic rifles such as AK-47s, along with grenades, rocket propelled grenade launchers, and Light Anti-Tank Weapons," or LAWs. The report's conclusions also draw straight from the source: MS-13 gang members themselves.

Worse, the authors write that the gang recently funneled multiple shoulder-fired SA-7 anti-aircraft missile launchers "to a clandestine arms market that operates in the Bajo Lempa region of El Salvador." According to the authors' sources, these SA-7s were first taken from stockpiles in Nicaragua left over from the country's civil war, transferred to the arms market in Bajo Lempa, and "were eventually sold to the FARC for use in Colombia." Going price? About $15,000.

Danger Room can't independently corroborate these claims, but the report notes an SA-7 was seized by Colombian troops in November. Last summer, the FARC also released a video showing what appears to be a rebel with an SA-7 launcher shooting down an airplane – likely a Colombian Air Force Super Tucano.

Ironically, the Maras' upgrade to automatic rifles is being inadvertently helped by the Salvadoran government. Gun buy-back programs, drawn up by the government to get guns out of the hands of street gangs, have allowed MS-13 to exchange old guns for cash, and then buy newer weapons – albeit in smaller quantities. Most of these weapons come from war-era stockpiles – many of them in Nicaragua – or by stealing (or even buying) them from the Honduran and Salvadoran armies. The MS-13 trade in firearms, whether for themselves or for selling them to larger and more organized groups like the FARC and the Mexican drug cartels, "appears to now be one of the primary activities of the MS-13 in El Salvador and beyond."

The report also details a growing link between the MS-13 and the Zetas – one of Mexico's most powerful and bloody cartels. This link hasn't always been well-established. An Associated Press story first made the connection last year, and quoted a Guatemalan interior ministry official who said the Zetas trained 18 MS-13 members at a camp in the Mexican state of Veracruz before sending them to Guatemala to conduct kidnappings. Days later, the chief of the Interior Ministry in Guatemala City dismissed the report, saying there was no evidence of a connection and denied the AP's source was an employee at the ministry.

But the connection does exist, according to Farah and Lum. Gang members told the authors that the Zetas and MS-13 reached an agreement "in the past few months" to arrange for MS-13 to smuggle drugs, weapons and human beings for the Zetas. MS-13 members are being trained at Zetas camps outside the Salvadoran capital of San Salvador, and then go on work for the cartel in Mexico for $400 per month. But the total numbers of MS-13 members trained "has not been so many," according to one MS-13 member. The monthly salary doesn't always go to the individual gangsters, instead being sent to the gangster's clique back in El Salvador.

This new partnership was also a way to patch-up some prior beef between the cartels. In 2010, the Zetas attacked MS-13 extortion gangs operating along railroad tracks in Mexico, where immigrants from Central America often travel to the U.S. border. The two sides fought for several months, with multiple people killed, until reaching a truce that divided up the territory. "We can't let ourselves be run off by anyone in our territory," said one MS-13 member. "We can work with anyone, but we won't work for anyone. That was what the fight was about."

It's very pragmatic-minded. MS-13 is also in a strong position to work with the Zetas, or pretty much anyone who wants illicit access to cocaine and automatic weapons. Their ability to do so, and cross borders with "relative impunity makes their organization ideal for the movement of guns," the authors note.