Cartel warriors now carry shoulder-fired launchers that can take down helicopters — and reach across the U.S. border. Should they be redefined as terrorists and rebels instead of just gangsters?

David Martinez Pelcastre / Reuters A federal police officer stands guard atop a vehicle as members of the investigative police team take genetic material from the parents of slain drug kingpin Heriberto Lazcano, outside a cemetery in Pachuca on Oct. 22, 2012.

When a Mexican SWAT team stopped a stolen Cadillac van in the border city of Piedras Negras, it was not a surprise when they were greeted by a tirade of bullets as the criminals blasted and ran. But after they kicked open the trunk, the officers realized they could have been victims of more catastrophic firepower. The gunmen had been in possession of an arsenal of weapons that included three Soviet-made antitank rockets complete with an RPG-7 shoulder-fired launcher. If the criminals had got a rocket off, they could easily have blown the SWAT vehicle to pieces. RPG-7s can also take out helicopters and were used in the Black Hawk Down episode in Somalia in 1993.

The rockets, found on Saturday, are part of an increasingly destructive array of weaponry wielded by Mexican drug cartels, like the feared Zetas, in reaction to attacks on them by police and soldiers. While security forces have taken down several key cartel bosses this year, gunmen have struck back, setting off five car bombs, hundreds of fragmentation grenades and several shoulder-fired rockets. Soldiers even seized one homemade three-ton tank with a revolving gun turret. When Mexican marines on Oct. 7 claimed to have killed Zetas leader Heriberto Lazcano, he was also alleged to be found with an RPG-7. (Lazcano’s corpse was stolen from the morgue, and the Zetas are now believed to be led by his No. 2, Miguel Treviño.)

The shoulder-fired rockets cause particular worry because of their range and explosive power. Mexican dignitaries often move in helicopters with the army flying Black Hawks supplied by the U.S. under the Mérida Initiative. “The RPG-7 is a weapon that causes incredible devastation from Iraq to Afghanistan,” says Rachel Stohl, an expert on arms proliferation at the Stimson Center in Washington. “When they fall into the hands of criminal groups, it changes the dynamics and escalates the conflict. Instead of just a gunfight on a street, you have military firepower.”

Combatants normally use RPG-7 rockets to target nearby vehicles, but they can reach up to 3,000 ft. (900 m) and are sometimes wielded as a form of artillery, scattering shrapnel at anyone close by. Those fearing spillover were quick to note that gunmen in Piedras Negras could potentially fire a rocket over the Rio Grande into the neighboring U.S. city of Eagle Pass. There have been sporadic gunfights across the river over the years, with gunmen recently firing at U.S. Border Patrol agents near the Texas town of Los Ebanos. (More often, Mexicans have been the victims, like when Border Patrol agents shot dead a 16-year-old boy in Sonora state this month.)

The gun trade has been a long-running bone of contention over the Rio Grande, with Mexico complaining that most of the firearms used by cartel assassins are purchased from U.S. stores. Of almost 100,000 guns seized at Mexican crime scenes since 2007, 68% have been traced to the U.S. The U.S. gun lobby argues that heavier weapons such as the Soviet rockets and fragmentation grenades come from the other direction, smuggled from Central America. Thousands of RPG-7s were used by all sides in the region’s Cold War conflicts in the 1980s. Since then, gangs have stolen many from lingering stockpiles in Nicaragua, El Salvador and Honduras to sell them on the black market. The Honduran government reports that it lost 22 RPG-7s and several rockets in a single 2010 theft. On a visit to Honduras earlier this year, a senior police officer said he had intelligence of Zeta operatives going to the capital, Tegucigalpa, to buy hardware.

When groups like the Zetas wield rockets and tanks, some pundits question whether they should continue to be labeled as drug traffickers — or need a more martial description. The cartel was founded in 1998 by 14 Mexican army defectors, and they carried their battle tactics into the crime world. “The Zetas are a criminal paramilitary organization that is spreading through Mexico and Central America like the bubonic plague,” says Mike Vigil, former head of international operations for the Drug Enforcement Administration.

The Zetas have used their firepower to make their stronghold in northeast Mexico, by the Rio Grande Valley, the country’s most violent corner. While other regions, like the area around Ciudad Juárez, have seen significant decreases in murders since 2010, Coahuila state, home to Piedras Negras, has witnessed its bloodiest year on record, with more than 640 gangland killings; neighboring Nuevo León has recorded over 1,000 such deaths since January. In total, almost 60,000 people have fallen in drug-related violence since President Felipe Calderón took power in 2006 and declared a military offensive on cartels. In the same period, 25 of Mexico’s 37 most wanted cartel bosses have been killed or arrested.

The buildup of cartel weaponry could also be a problem for incoming President Enrique Peña Nieto when he takes office in December. Peña Nieto, who returns the Institutional Revolutionary Party, or PRI, to power after 12 years in the wilderness, has promised to halve the number of homicides in Mexico in his first year. While all applaud the target, Peña Nieto has given little concrete information about how he will achieve the goal. When Zeta squads roam the countryside with RPG-7s, some say Peña Nieto could be forced to continue a military line similar to Calderón’s. “There will probably be a change in rhetoric,” says political analyst Jorge Chabat, “but there is a little room for maneuver in tactics.”