Hours after he was charged with hacking a teenager's hands with a machete, Hayner R. Flores etched one of the most feared symbols in Northern Virginia on the glass window of his Fairfax County jail cell door.

The 18-year-old Salvadoran immigrant, sheriff's deputies said, scrawled "MS-13,'' the name of a street gang notorious for its violence. It was a defiant and brazen gesture, qualities that are emblematic of MS-13, or Mara Salvatrucha. The 16-year-old victim of the May 10 assault, police said, was a member of a rival gang, South Side Locos.

The machete attack, followed by the fatal shooting of a Herndon teenager by an assailant with an MS-13 tattoo on his forehead, have served as a wake-up call to the community about what police said has been a rapidly growing, and increasingly violent, problem in recent years.

Police in Northern Virginia universally point to MS as the most dangerous and fastest growing gang, with more than 1,200 members. MS-13 members have been linked to at least seven slayings in Northern Virginia in the past four years, including a federal witness, as well as numerous rapes, beatings and property crimes.

According to law enforcement officials, MS-13 is an international criminal enterprise with roots in El Salvador's civil war. They say it has established a foothold in Northern Virginia and Montgomery and Prince George's counties and in predominantly Latino neighborhoods in the District.

The gang's Northern Virginia members are split into subgroups, or cliques, that hang out and often hold mandatory meetings in parks -- sometimes disguised as soccer games -- but there is still little organized criminal activity, such as drug trafficking. They call themselves a family, collecting "dues" to post bail for a jailed member, yet they beat members who miss a meeting or fail to stand up to a rival gang. Unlike other crime organizations, police said, their lack of structure and hierarchy make them difficult to dismantle.

Bred in a culture of violence in their war-torn homeland of El Salvador, the largely Latino gang has been seeking to establish an East Coast hub in the Northern Virginia suburbs for a decade, police said. The first signs of trouble were spray-painted gang graffiti and boasting about gang affiliations, but things soon turned violent.

In 2001, Fredy Reyes-Castillo, 22, was beaten to death by four MS-13 members in a Reston park. The same year, a young mother was raped and killed behind her Seven Corners apartment and a reputed MS-13 member is awaiting trial on a capital murder charge in her death. In Prince William County, four Mara Salvatrucha members were arrested in 2002 after a 17-year-old told police she was "sexed in" -- initiated by having sex with six members.

"They have a violent reputation, and they want to maintain that," said James O. Towey, a Virginia assistant attorney general who has studied the gang.

A February memo to U.S. Attorney General John D. Ashcroft from Paul J. McNulty, the U.S. attorney in Alexandria, said that gangs are a growing problem in Northern Virginia and that MS-13 is "by far the gang of greatest concern." Those worries have been echoed often by police, parents and school and community officials after the two recent episodes of violence.

Last week, a 17-year-old Herndon youth was shot to death by a bicycle-riding assailant who has "MS" tattooed on his forehead, police said. No arrests have been made, and law enforcement sources said the victim, Jose Sandoval, may have been a member of the rival 18th Street gang. A 16-year-old girl who was with Sandoval was shot in the back and is recovering.

The shooting came less than a week after the 16-year-old Fairfax boy was attacked by several people, one wielding a machete, as he walked along Edsall Road early on a Monday morning. Police said the victim is a member of South Side Locos, a relatively new but growing multiracial gang that may have been formed by a disgruntled MS-13 member.

Just hours after he was charged with malicious wounding and participating in a street gang, Flores scratched "MS-13" and his nickname, Spike, into the window of his jail cell, Fairfax sheriff's officials said. They believe he used a zipper or a small piece of metal that is part of a plastic bracelet inmates wear. He was charged with destruction of property.

Although police said the latest attacks were unconnected, they worry that the sudden burst of brutality, possibly motivated by rival gangs competing for dominance in the suburbs, could bring even more violence.

"Everyone is on a heightened sense of alert, but you can't really predict where it's going to happen," said Scott Mastandrea, a Loudoun County detective who is a member of a Herndon-based gang task force, made up of Northern Virginia gang investigators.

Largely in response to the gang's violence, state lawmakers have tightened laws targeting gang members and increased penalties for gang crimes. The FBI has made MS-13 and other violent gangs a priority, and federal prosecutors in Virginia opened a wide-ranging grand jury probe aimed at dismantling the gang.

Law enforcement officials said that perhaps the biggest, and most important, challenge is keeping them from growing. MS members have been recruiting younger and younger children, many from troubled homes, who choose the protection of the gang over fear of becoming one of its victims. Fairfax police have said children as young as 7 have been found with MS-13 graffiti on their notebooks.

New members, mostly teenagers, are drawn to gang life by the promise of companionship and status and often think of it as a family, gang investigators and school officials said. Many new recruits are recent immigrants seeking a sense of belonging in the community. Among the most powerful attractions, according to several law enforcement officials, is the promise that plenty of girls hang out with the gang.

"It offers you identity. It offers you status, and it offers you plenty of sex," one gang detective said. "You ask the kids why they join and they say: 'I like the way they carry themselves. I like the way they talk. I like the way they walk.' "

Mara Salvatrucha took root in Los Angeles during the mid-1980s with young men who fled the violent civil war in El Salvado, many of them having received combat training, police said. A little more than a decade ago, members began migrating to Northern Virginia, drawn by the established Salvadoran population. Even today they closely follow news of gang-related violence in their homeland, investigators said. They saw Northern Virginia as an appealing place where they easily could dominate loosely knit homegrown gangs. Members initially settled in the Culmore neighborhood near Baileys Crossroads but have expanded to Alexandria and Arlington, Prince William and Loudoun counties. More recently, Mara Salvatrucha's members have been moving west into the Shenandoah Valley and south toward Charlottesville, federal officials said. State officials said they have seen evidence of the gang in Danville and Richmond.

Although Mara Salvatrucha has been known to deal drugs, sell guns and steal cars, there is little structured criminal activity and a loose leadership structure, making it difficult to dismantle the gang by targeting top members, law enforcement officials said. Police said there are signs that the gang is seeking to become more organized, with MS-13 leaders in other cities, such as Los Angeles, encouraging more structure here.

But for now, violence is its hallmark.

"Out in California they are in the business to make money, but here they are in it to make a name for themselves," Mastandrea said. "Some guys are out to make money. Some guys are in it for the thrill of it. Some guys are in it for protection. Some of them don't see any other alternative."

Mara Salvatrucha's structure is so loose, one federal law enforcement source said, that every member of a clique is considered equally likely and willing to carry out violent acts. That makes MS-13 more violent and potentially harder to contain than, for example, Mafia groups that have clear leadership structures and lines of authority, he said.

"There's not a big disparity between the top and bottom," the source said. "That makes them more difficult to combat. You create a vacuum by going after someone, and a person with the same skill set can move up and fill the position."

Sudden and vicious attacks can be sparked by the smallest of slights, either real or perceived, gang investigators said.

In the June 2001 slaying in the Reston park, MS-13 members beat Reyes-Castillo and left him to bleed to death simply because he pretended to be a member of their gang, Fairfax prosecutors said. Three months later, Mara Salvatrucha members lured a man to an Alexandria park and nearly beheaded him because they mistakenly thought he belonged to a rival gang, federal prosecutors said.

"No disrespect goes unchallenged," said one local law enforcement source. "It could be that you looked at someone the wrong way. It could be that you went on their turf."

Knives and baseballs bats have been their weapons of choice, but police are seeing more handguns.

"Right now, the extent of their activities is something we are really trying to get our arms around,'' said Michael A. Mason, assistant director in charge of the FBI's Washington field office. Last fall, Mason created a new squad of agents to combat what he called rising gang violence in Northern Virginia and the District.

"It is imperative that law enforcement not wait until groups like MS-13 can grow roots and solidify . . . and develop that organizational structure that would make it more potent than it is today,'' Mason said.

Hayner R. Flores, 18, scratched "MS-13" into his jail cell window, Fairfax sheriff's officials said.Prince William police said they seized this MS-13 plaque in a raid.