A member of the Mara Salvatrucha street gang pleaded guilty yesterday to federal charges that included the slaying of a Herndon teenager, marking the first conviction in a regionwide effort to target the violent gang using broad federal racketeering laws.

Osmin Heriberto Alfaro-Fuentes admitted that he was involved in the May 2004 slaying of Jose Sandoval, 17, a killing that heightened awareness of the Washington area's growing problem with such gangs as Mara Salvatrucha, or MS-13.

Alfaro-Fuentes said he and fellow MS-13 member Alirio Reyes confronted Sandoval and a 16-year-old female companion on a Herndon street. When they realized Sandoval was a member of a rival gang, Reyes shot and killed Sandoval and shot and wounded the girl, Alfaro-Fuentes said.

"The rules in MS-13 are that if you face a member of a rival gang, there has to be a fight and there may even be a death," Alfaro-Fuentes said as he pleaded guilty in U.S. District Court in Alexandria. A large tattoo, reading "MS" in large green letters, was emblazoned across his forehead.

Law enforcement officials said Alfaro-Fuentes is the first MS-13 member convicted in the Washington region on federal racketeering charges, which have long been used to combat more traditional organized crime but are increasingly being employed nationwide against violent street gangs. Federal prosecutors in Maryland charged 19 MS-13 members last week with racketeering counts, the first such case brought there, after a surge of gang violence.

Yesterday's plea requires Alfaro-Fuentes's cooperation with prosecutors, and he could provide significant information about MS-13's organization and structure.

Reflecting the potential danger Alfaro-Fuentes faces, U.S. District Judge T.S. Ellis III ordered that he be housed in prison away from other members of the gang. "He can't be put in the same area with any MS-13 members," Ellis said.

Alfaro-Fuentes is expected to testify against Reyes, who has pleaded not guilty. Alan Yamamoto, an attorney for Reyes, said his client "denies being there" when Sandoval was shot and "denies being any part of this." He said Reyes plans to go to trial.

Alfaro-Fuentes, who pleaded guilty to one count of murder in aid of racketeering, faces up to life in prison when he is sentenced Dec. 9. The crime carries a potential death sentence, but Attorney General Alberto R. Gonzales decided not to pursue the death penalty, law enforcement officials said.

Sandoval and the girl were shot as they walked in the area of Cavalier Drive and Park Avenue in Herndon. Court documents say that Alfaro-Fuentes questioned Sandoval about his gang affiliation and that Sandoval said he was a member of the Los Angeles-based 18th Street gang, the chief rival of MS-13. Reyes then shot the pair with a .38-caliber weapon, the documents say.

The girl, who was shot in the back and hospitalized for 10 days, told investigators that the man who fired the shot was on a bicycle and had "MS" tattooed across his forehead, authorities have said. A second man was on foot.

Less than a week earlier, a gang-related machete attack on a 16-year-old boy occurred in the Alexandria area of Fairfax County. Collectively, the episodes triggered a flurry of interest from politicians and led to more law enforcement task forces and increased anti-gang education efforts in the region.

Alfaro-Fuentes and Reyes were indicted on racketeering and other counts in December.

The federal racketeering statute, known as the RICO law, was enacted by Congress in 1970 to give law enforcement a powerful tool against the Mafia. For several decades, it was used primarily to bring down organized crime leaders by convicting them of being part of broad criminal enterprises.

But in recent years, authorities have used racketeering laws to successfully prosecute street gang members from Los Angeles to Utah. The charges against Alfaro-Fuentes and Reyes were the first time the statutes had been used in Virginia.

Court documents filed with yesterday's plea characterize MS-13, which has its roots in El Salvador and Los Angeles, as a broad criminal enterprise. MS-13 members travel throughout the United States to attend gang meetings, wire money across state lines for gang-related purposes and engage in crimes ranging from homicide to assault and auto theft, the documents say.

The court papers say Alfaro-Fuentes was involved in the killing of Sandoval to increase his "status" within the gang.

Jose Sandoval, 17, of Herndon was killed in 2004.