The signs of a new threat in northern Virginia emerged ominously in blood-spattered urban streets and rural scrub. Two summers ago the body of a young woman who had informed against her former gang associates was found on the banks of the Shenandoah River, repeatedly stabbed and her head nearly severed. Last May in Alexandria, gang members armed with machetes hacked away at a member of the South Side Locos, slicing off some of his fingers and leaving others dangling by a shred of skin. Only a week later in Herndon, a member of the 18th Street gang was pumped full of .38-caliber bullets, while his female companion, who tried to flee, was shot in the back. The assailant, according to a witness, had a large tattoo emblazoned on his forehead. It read MS, for Mara Salvatrucha, the gang allegedly responsible for all these attacks.

At the nearby headquarters of the Federal Bureau of Investigation, agents--many of whom live in these communities--fielded the reports with mounting alarm. But Mara Salvatrucha, or MS-13, wasn't terrifying just northern Virginia. "They were popping up everywhere," says Chris Swecker, assistant director of the FBI's criminal investigative division. "It seemed like we were hearing more and more about MS-13." Then one day last fall, FBI Director Robert Mueller called Swecker into his office. "You have a mandate to go out and address this gang," Mueller told him. Mueller declared MS-13 the top priority of the bureau's criminal-enterprise branch--which targets organized crime--and authorized the creation of a new national task force to combat it. The task force, which includes agencies like the Drug Enforcement Administration (DEA) and Immigration and Customs Enforcement (ICE), aims to take on MS-13 much as the FBI once tackled the Mafia.

Composed of mostly Salvadorans and other Central Americans--many of them undocumented--the gang has a uniquely international profile, with an estimated 8,000 to 10,000 members in 33 states in the United States (out of more than 700,000 gang members overall), and tens of thousands more in Central America. It's considered the fastest-growing, most violent and least understood of the nation's street gangs--in part because U.S. law enforcement has not been watching as closely as it might have. As authorities have focused their attention on the war against terrorism, MS-13 has proliferated. In the FBI's D.C. field office, the number of agents dedicated to gang investigations declined by 50 percent. "There was a definite shift in resources post-9/11 toward terrorism," says Michael Mason, assistant director in charge of that office. "As a result, we had fewer resources to focus on gangs," though he adds that the bureau made up for any shortfall by leveraging resources from other agencies. In recent weeks, authorities have made strides against MS-13: a gang leader accused of orchestrating a December bus bombing in Honduras that killed 28 people was arrested in Texas in February, and a recent seven-city sweep by ICE netted more than 100 reputed MS-13 members. But Robert Clifford, head of the new national task force, says "no single law-enforcement action is really going to deal the type of blow" necessary to dismantle the gang. No one is more interested in busting up MS-13 than leaders of the Latino community, who live with the fear and fallout of the gang's savage actions.

MS-13 got started in Los Angeles in the 1980s by Salvadorans fleeing a civil war. Many of the kids grew up surrounded by violence. Del Hendrixson of Bajito Onda, a gang-outreach program, remembers an MS-13 member who recounted one of his earliest memories: guarding the family's crops at the age of 4, armed with a machete, alone at night. When he and others reached the mean streets of the L.A. ghetto, Mexican gangs preyed on them. The newcomers' response: to band together in a mara, or "posse," composed of salvatruchas, or "street-tough Salvadorans" (the "13" is a gang number associated with southern California). Over time, the gang's ranks grew, adding former paramilitaries with weapons training and a taste for atrocity. MS-13 eventually adopted a variety of rackets, from extortion to drug trafficking. When law enforcement cracked down and deported planeloads of members, the deportees quickly created MS-13 outposts in El Salvador and neighboring countries like Honduras and Guatemala.

Flush with new recruits from Central America, whether fleeing the law or accompanying parents seeking work along the immigrant trail, MS-13 members have set up cliques--geographically defined subgroups--in such remote redoubts as Boise, Idaho, and Omaha, Neb. In these new settings, gang culture often morphs. "Everything gets bastardized as it leaves the center," says Wes McBride, president of the California Gang Investigators Association. While machete attacks might occur on the East Coast, they're rare on the West Coast. While car thefts and drug trafficking might be big in North Carolina, gang-on-gang violence predominates in Virginia. It's that decentralized nature of MS-13--with no clear hierarchy or structure--that makes it so vexing to authorities. "Taking out the heart of the leadership is very hard if there is no definitive leadership," says one federal law-enforcement official.

But that could be changing. According to a 2004 report by the National Drug Intelligence Center, the gang "may be increasing its coordination with MS-13 chapters in Los Angeles, Washington, D.C./Northern Virginia, and New York City, possibly signaling an attempt to build a national command structure." One potential illustration of such an effort: on New York's Long Island last year, an MS-13 honcho arrived from the West Coast "to try to organize these various cliques or sets into a more formal structure," says Robert Hart, supervisory special agent with the FBI. "That's a significant step in the development of MS-13." And in northern Virginia, U.S. Attorney Paul McNulty observes that "in some of the violent crimes, there seems to be a kind of approval process in some kind of hierarchy beyond the clique."

If MS-13 is seeking to create a national command in the United States, it would be emulating its model in El Salvador. There, says Oscar Bonilla, director of the National Council for Public Security, the gang is "highly organized and disciplined... with semi-clandestine structures and vertical commands." As a result, its criminal operations are all the more efficient and pervasive. The administration of President Tony Saca has responded with a super mano dura ("super hard hand") policy, reforming the penal code to facilitate gang prosecutions. "We're not dealing with Boy Scouts or bums," Saca told NEWSWEEK. "We're dealing with true assassins, rapists."

In the United States, Clifford's new national task force, which will be housed at FBI headquarters, is preparing a hard hand of its own. Serving as a national repository for MS-13 intelligence, it will help discern trends, prioritize targets and diagram whatever leadership structure might exist. There's an international dimension, too: U.S. investigators will be exchanging information--such as a gang member's movements and associates--with their counterparts in Central America. FBI agents sitting in regional U.S. embassies will serve as liaisons with local authorities, and Salvadoran advisers will come to the United States to share their MS-13 expertise. All of which amounts to "a comprehensive international attack against MS-13," says Clifford.

But some kinks remain. In the recent sweep conducted by ICE, the agency nabbed a gang member whom the FBI was intensely interested in. "This was not somebody we were ready to scoop up," says a federal law-enforcement official, who complains that ICE didn't alert other agencies of its impending raid. (An ICE spokeswoman insists that all targets were cleared with other agencies. Another ICE official grumbles that "the bureau thinks it has jurisdiction over everything.") Meanwhile, down in El Salvador, officials fear the repercussions of another batch of MS-13 deportees heading their way. "Those deportations are a time bomb," says Bonilla. "When a gang member is deported from the United States, it destroys in one month what we've achieved in a year of [gang-prevention work]." For authorities to succeed in this war, they'll need to cooperate at least as well as the gang they're trying to wipe out.