A Fairfax County teenager has been charged in the machete attack of a 16-year-old boy, and sources said police are investigating whether a fight between rival gang members at a carnival at the Potomac Mills mall 10 days earlier was related to the assault.

Hayner R. Flores, 18, a reputed member of the violent and well-established Mara Salvatrucha street gang, or MS-13, was arrested about 10 p.m. Wednesday at his Annandale apartment, police said yesterday. He was charged with malicious wounding and participation in gang activity.

Police said Flores was among a group of gang members who assaulted the teenager, a member of rival gang South Side Locos, or SSL, as he walked along Edsall Road about 1 a.m. Monday.

The victim's hands were mutilated in the attack. All four fingers on his left hand were sliced off and his right hand was nearly severed, his doctor said. Police said the youth also had other unspecified wounds.

Fairfax County's acting police chief, Lt. Col. Suzanne G. Devlin, said yesterday that the department will be "really aggressive about prosecuting the people involved in this case," in part as a message to other gang members. In an effort to quell further acts of revenge between the two gangs, Devlin plans to continue stepped-up patrols in areas where gang members are known to gather.

Police said Monday's attack was the latest incident in a feud between Northern Virginia's two largest gangs. Arlington County police said an April 2003 fatal stabbing that occurred during a party at an Econo Lodge motel also stemmed from a dispute between MS-13 and SSL, a newer but growing gang that was founded by a former Mara Salvatrucha member.

Devlin said yesterday that it is unclear whether the wounded teenager was targeted or whether MS-13 members were seeking revenge against any SSL member.

A police source said detectives are investigating whether the assault was sparked by a confrontation between the two gangs that occurred 10 days earlier during a carnival at Potomac Mills in Prince William County.

Just before midnight April 30, the victim in the machete attack was at the carnival with fellow SSL members when a member of his gang punched a purported MS-13 gang member in the face, according to court records.

Prince William Detective John T. Mora said in court papers that he arrested the SSL member, Guillermo Amaya-Amaya, 18, and charged him with assault and battery.

Mora said Amaya-Amaya told him that he was not a gang member -- that he just hangs out with SSL. But in the arrest warrant, Mora said Amaya-Amaya was a member of SSL and charged him with participating in a street gang.

Amaya-Amaya told the officer that he threw the punch after the rival gang member "bumped his friend" and then flashed the MS-13 hand sign, Mora wrote in the papers filed in Prince William Circuit Court.

Fairfax police said the tension between the gangs has been evident in recent months as SSL's ranks increase across the region.

Mara Salvatrucha, which was formed by young men from war-torn El Salvador, started in Los Angeles but made Northern Virginia its East Coast hub, gang experts said.

Authorities estimate that there are 1,200 to 1,500 members locally, and the gang has been linked to at least five slayings in Virginia, including the killing of a federal witness, in addition to baseball bat beatings and rapes.

South Side Locos was organized in the area a little more than two years ago by a former Mara Salvatrucha member whose uncle was killed by another MS-13 member and has recruited members quickly, according to Sgt. Greg Smith of the Fairfax police gang unit.

Police said that while MS-13 primarily attracts Latino members, SSL is multiracial.

Authorities estimate that SSL's ranks have grown to about 400 members.

Mora said in court documents that SSL members have been charged with violent crimes and drug offenses.

Devlin said yesterday that the investigation is continuing and that additional arrests are likely. But she stressed that arrests in this case will not solve the problem and encouraged teenagers to come forward with information about gangs.

"Its really time for the youth in Fairfax County to say we can no longer afford to be silent," Devlin said.

"We need kids to step up and take a leadership position," she added.

As police continue to search for suspects, the wounded 16-year-old, a student at Richard M. Milburn High School, an alternative school, faces a long and painful recovery, said his surgeon, Khalique S. Zahir at Inova Fairfax Hospital.

Zahir said he expects that after several months of rehabilitation, the youth will regain nearly complete use of his right hand and that he will be able to move his left thumb.

Virginia Attorney General Jerry W. Kilgore (R), who has been hosting a series of community meetings on gang violence and has pushed for new laws to combat gangs, said the attack is a reminder that more needs to be done to thwart gang activity.

He said he's concerned that the rivalry between MS-13 and SSL could escalate.

"MS-13 has dominated for so long in Virginia that having SSL come in ratchets this up a lot," Kilgore said. "This ought to be a wake-up call to the community and to parents."

Hayner R. Flores, 18, of the Annandale section of Fairfax County, is charged in Monday's machete attack on a 16-year-old.