The attack on a 16-year-old Fairfax County youth whose hands were mutilated with a machete was an act of revenge in an ongoing dispute between the two largest gangs in Northern Virginia, police said yesterday.

The victim, who is recovering from his wounds at Inova Fairfax Hospital, had tattoos on his hands indicating that he was a member of the South Side Locos, a relatively new gang in Fairfax that has grown quickly in the last two years. Investigators said they believe that the assailants were members of Mara Salvatrucha, or MS-13, the dominant street gang in Northern Virginia, which has been linked to numerous slayings and assaults in recent years.

"This was an assault that was in retaliation for a previous assault, which was in retaliation for a previous assault, and so on," said Sgt. Greg Smith of the Fairfax police gang unit. "This has been an ongoing thing since SSL [South Side Locos] came into existence. In the big picture, with what's gone on with MS and SSL over the last two years, this is just another day in the office."

No suspects were in custody yesterday, although police said they were pursuing promising leads.

Monday's attack is part of a growing gang problem in Northern Virginia that law enforcement officers have been struggling to contain for about four years. But gang membership continues to soar, and police estimate that there are 2,500 members in the area, holding them responsible for rapes, beatings, stabbings and the killing of a witness in a federal murder investigation. A regional gang task force, formed in Herndon last summer, made 36 felony arrests in its first 90 days, according to the Loudoun County sheriff's office.

"It will probably get worse before it gets better," Loudoun Sheriff Stephen O. Simpson. "We have been saying all along these are not the homegrown groups like you used to see. If left alone, it has the potential to get out of control."

Police said the South Side Locos now have about 400 members in the Fairfax area, recruited in a little more than two years. Smith said the gang was organized in the area by a disgruntled former Mara Salvatrucha member whose uncle was killed by another MS-13 member in Arlington. Mara Salvatrucha started in Los Angeles in a community of immigrants from El Salvador.

"They are organized," Smith said of the South Side Locos, though not to the extent of MS-13, which is estimated to have 1,200 to 1,500 members in the area. "This is not just 400 guys loosely running around Fairfax County. They do have meetings. They have gotten so large that we're hearing they're breaking into [smaller units], like MS."

About 1 a.m. Monday, the 16-year-old was attacked by several people as he walked in the 6400 block of Edsall Road in the Alexandria area of Fairfax. Yesterday, police said that neither of the boy's hands was severed, contrary to their original report.

The boy's left hand was slashed through the knuckles, and only his thumb could be saved, according to his surgeon, Khalique S. Zahir. The teenager's right hand was nearly severed, but Zahir said he reattached the thumb and three fingers by reconstructing the bones, blood vessels and tendons.

Zahir, a plastic and reconstructive surgeon, operated on the teenager for about 71/2 hours Monday. He said the teenager is recovering well despite the enormous blood loss and the severity of the injuries. Zahir said that he has treated other victims of gang-related machete attacks but that Monday's was among the most brutal.

"It's a very horrible thing," Zahir said. "The people who did this did it not for him to die, they did if for him to suffer, and that's evil."

Officer Bud Walker, a Fairfax police spokesman, said the victim suffered cuts to his upper body in addition to the hand wounds. Walker said the hand injuries could have been received as the teenager defended himself.

Smith said that the hands of victims had been severed or sliced by machetes in other gang attacks but that the intent was not to wound the hands specifically. "In this particular case, I can't tell you" if the victim's hands were targeted. "Was it deliberate? I kind of think not."

The 16-year-old was a quiet and unassuming student who sported gang tattoos and colors at Richard Milburn High School, an alternative school of about 55 students near Alexandria, according to teachers.

The teenager is known for wearing a red or blue bandanna, which he was required to remove during class, and for having the South Side Locos gang symbols imprinted on the knuckles of one hand.

Deborah C. Brown, the school's director, said that about a month ago, the teenager came to her with a gang-related problem. He told her that he could no longer share his taxi ride to school with another student because he had discovered that the student was a member of a rival gang. Brown said she fixed the problem by arranging for another taxi to shuttle him to school.

Brown said she did not urge him to abandon his gang activities, because "I knew that his mother and a probation officer were talking to him about it. You don't want too many cooks in the kitchen."

The school's security chief, Leroy Washington, said the boy did not have disciplinary problems and was very reserved, often pretending not to understand English when he really did. "He would play the 'I Can't Speak English' game, but he spoke good English when he wanted to," Washington said. "He would doodle on his folder a lot, a knife or their gang symbols."

Smith said the name South Side Locos has its origins in Southern California, where it is an established street gang, also formed by a disaffected former MS member. Similarly named gangs have sprung up in cities across the west, but Smith said, "We believe that our SSL guys are pretty much homegrown." He also said the Northern Virginia version is multiracial, not just restricted to Latinos, and recruits relatively young members, mainly high school- and even middle school-age males.

The gang has been involved in armed robberies, assaults and carjackings, Smith said, including robberies of pizza deliverymen in the Mount Vernon area. But SSL's main crime is graffiti. "This is part of making their mark," Smith said.

Police said most of SSL's violent crimes have been directed against MS-13. "They have been assaulting one another ever since [SSL] was formed," Smith said.

"For the longest time," Smith continued, "MS treated these guys like the annoying mosquito in the room," partly because of the younger membership. "But SSL kept ratcheting up the attacks. MS has finally answered, obviously."

Smith added: "SSL will retaliate."

In another case, an SSL member was killed last month in a shooting outside a Days Inn motel along Route 1, allegedly by members of a gang called La Primera. Smith said SSL probably would retaliate for that incident, too. "They've got a couple of things on their plate," Smith said of the gang.

Staff writers Maria Glod and Elaine Rivera and staff researcher Bobbye Pratt contributed to this report.