MS-13, shorthand for "Mara Salvatrucha," is one of the world's most lethal gangs, with a power and reach that exceeds that of some national governments. It has ravaged the tiny Central American country of El Salvador, and its influence extends into neighboring Honduras and elsewhere.

But MS-13 isn't a homegrown Salvadoran phenomenon. It's an export from Los Angeles, where many gang members were initiated as adolescents and young adults, before being deported back to El Salvador and taking their violent methods with them. Today, as depicted in the new documentary "Gang Warfare USA," airing at 8 Monday night on the National Geographic Channel, MS-13 members in El Salvador work with their U.S. counterparts to export violence to cities as remote from L.A. as Greensboro, N.C.

Marc Shaffer, the film's director, producer and writer, and his crew detail the disturbing story of how a restaurant murder in Greensboro eventually led investigators to L.A. and El Salvador. Along the way, they uncover how Uncle Sam's deportation of MS-13 members to El Salvador ironically has been making the gang even stronger and more globalized than before.

In interviews with current and former gang members, U.S. Immigration and Customs Enforcement officials, attorneys and others, the documentary exposes that many gang members deported to El Salvador, where economic prospects are bleak, soon turn right around and cross back into the United States.

Meanwhile, the gang's presence in El Salvador continues to undermine the rule of law in that war-torn country: El Salvador, with a population of only 6 million, has a murder rate 10 times that of the United States, and officials estimate that 70 percent of those murders are gang-related. As one assistant U.S. attorney tells the filmmakers, "We set up the conditions by which MS-13 flourished."