An assailant on a bicycle who gunned down a 17-year-old Herndon youth Sunday night was sporting an "MS" tattoo on his forehead, signifying allegiance to Northern Virginia's largest street gang, law enforcement sources said yesterday.

Jose Sandoval was shot several times as he walked with a 16-year-old girl through a Herndon neighborhood. The girl, who has not been identified by police, survived a gunshot wound to the back, Herndon police said.

The tattoo on the gunman's forehead, reported to police by a witness, is a symbol of the gang Mara Salvatrucha, or MS-13. The Latino gang is thought to have more than 1,200 members in Northern Virginia alone and has been implicated in numerous homicides and other violent crimes.

Gang experts said the higher an MS tattoo is placed on the body, the more violent the crimes the person has committed. Tattoos above the neck indicate that a member has assaulted or killed a rival gang member or a police officer, they said.

Students at Herndon High School, where Sandoval was a freshman, wore black clothing yesterday and held a vigil in his memory. They painted a large rock in front of the school -- one that is typically used to celebrate events -- with the words "RIP SLeepy" and surrounded it with flowers. "We MISS U" was written on the back.

Sandoval apparently earned the nickname because his quiet nature convinced his friends that he was sleeping all the time.

Also yesterday, Virginia Attorney General Jerry W. Kilgore (R) met with members of Fairfax County's police gang unit to offer them more prosecutorial support and to announce a Gang Reduction Initiative Program. Kilgore said the program would enable police to seek federal charges and use federal grand juries against gangs operating across county lines.

Authorities in the region were rushing this week to devise new strategies to combat a seemingly rising tide of gang-related bloodshed. Last week, in the Alexandria area of Fairfax County, the hands of a 16-year-old reputed gang member were mutilated, allegedly by a machete-wielding 18-year-old who police believe is an MS member. Investigators have not found any connection between the May 10 machete attack and the killing of Sandoval.

But the gangs that were victimized -- the machete victim is a suspected member of South Side Locos, and Sandoval is believed to have been in a gang called 18th Street -- are expected to retaliate against MS.

"We're extremely concerned about it," said Sgt. Greg Smith of the Fairfax police gang unit. "It's our biggest fear right now. . . . With the gang mentality, that crime must be answered," meaning that gangs inevitably seek revenge for crimes committed against their members. He added, however, that "oftentimes the retaliation doesn't come right away. Sometimes it's six months."

Herndon police declined to release any information about Sandoval's slaying yesterday. Police plan to hold a community meeting tomorrow night at Herndon's town hall to answer questions from residents.

Also tomorrow, Rep. Frank R. Wolf (R-Va.), accompanied by federal prosecutors and law enforcement officials, will hold a news conference to announce increased U.S. resources to combat gang violence. Wolf helped raise $500,000 in Congress last year to launch a regional gang task force, now based in Herndon, and Kilgore said yesterday that the task force would have to be funded annually.

Kilgore noted that Virginia's General Assembly passed a number of laws this year, to take effect July 1, that penalize gang recruitment and involvement more heavily. He said he learned from Fairfax gang officers that the state needs to improve its ability to protect witnesses in gang cases. "We need to make that a priority," Kilgore said.

Fairfax police reported more than 700 gang-related incidents last year. Many were related to graffiti, but statistics also show 37 serious assaults, 55 simple assaults, 13 robberies and seven rapes attributed to gang members. One alleged MS member is awaiting trial on capital murder charges in the 2001 rape and killing of a woman in the Falls Church area.

"This is nothing new," Smith said. "MS commits crimes every day. We've been dealing with this for years."

Another MS member with a forehead tattoo, Marvin Campos, was successfully prosecuted in Fairfax in 2000 for his role in recruiting and supervising a 14-year-old MS gang member who stabbed a man to death that summer. The youth, serving a 23-year prison sentence, is now covered with MS tattoos.

Smith said poverty or lack of education, which make gangs more attractive elsewhere, are unlikely causes of gang affiliation in Fairfax, one of the nation's wealthiest counties. He attributed it to the marketing of gangster culture, through music, clothes and language, as the hip route for rebellious youngsters. "I think it's become almost fadlike," Smith said. He urged parents to become more knowledgeable about their teenagers' lives.

Police believe Sandoval claimed loyalty to the gang 18th Street, but his family could not be located yesterday to confirm or deny that. A 16-year-old sophomore, who declined to be identified because he feared gang retaliation, said he was one of Sandoval's best friends. Their families would get together at a park on Sundays for a barbecue of hamburgers and hot dogs and play volleyball and basketball.

He said Sandoval's "mother is really hurting. She didn't even want to talk to my mom," the youth said.

"He was a nice person, wouldn't get in any trouble and practically had no enemies. He was tight," he said. "I don't think he was involved in a gang. Maybe [the assailant] got confused or something." Sandoval wanted to be an artist when he grew up, the friend said.

Staff writer Elaine Rivera contributed to this report.

Virginia Attorney General Jerry W. Kilgore met with members of Fairfax County's police gang unit and offered them more prosecutorial support.