An assailant on a bicycle shot and killed a 17-year-old Herndon youth and wounded a 16-year-old Sunday night in what police said yesterday was another explosion of gang-related violence in Northern Virginia.

Police said Jose Sandoval, a freshman at Herndon High School, was killed about 9:40 p.m. as he walked on a quiet Herndon street, not far from the popular Washington & Old Dominion biking and hiking trail. Neighbors said they heard a rapid burst of shots and then saw the teenage girl standing over Sandoval, screaming.

"We're pretty confident that this is gang-related," said Sgt. Jerry Keys of the Herndon Police Department.

Three law enforcement sources said the assailants were believed to be members of Mara Salvatrucha, or MS-13, a violent street gang in Northern Virginia thought to have more than 1,200 members. The victim was suspected of being a member of a rival gang called 18th Street and was confronted by two MS-13 members, one on a bike and the other on foot, the sources said.

The shooting comes as police were preparing for more gang violence after a machete attack on a 16-year-old boy in the Alexandria area of the county. Gang-related attacks have been growing more violent and prevalent and have been all over Northern Virginia, with slayings, rapes, beatings and stabbings spreading from Arlington across Fairfax and into Loudoun and Prince William counties.

Sandoval, shot multiple times, was taken to Reston Hospital Center and pronounced dead. The girl, who was not identified by police, was shot in the back as she ran away, sources said. She was in stable condition yesterday at another hospital. The Washington Post is not providing additional information about the girl because police said doing so would endanger her.

Investigators saw no sign that the case was related to the machete attack on the 16-year-old boy. In that attack, police arrested an 18-year-old man suspected of also being a member of MS-13, and police said the victim was a member of South Side Locos, a rival gang.

The victim lost four fingers on one hand in the attack, and his other hand had to be sewn back together. His parents denied he was in a gang, though schoolmates and administrators said he clearly indicated gang loyalty.

The homicide was the first in Herndon in more than three years, since Richard W. Hamilton shot his wife, two children and then himself in November 2000. Keys said the neighborhood where Sunday's shootings occurred, in the 1000 block of Park Avenue on Herndon's north side, averaged fewer than 10 calls for service per month this year, which he said was about average for the town of 22,000.

Residents on the street of townhouses, built in the mid-1970s, said they heard three to six shots Sunday night. "At first, I thought it was firecrackers," said Wayne Jenkins, a 28-year resident. "Then I said, 'That's a gunshot.' I looked out the window, and the guy was laying out in the street. The girl was coming toward him. He was lifeless. The girl was standing over him screaming. Then she moved over to the grass."

A resident, who would speak about the incident only if her name were not used because she is scared, said she and her children heard four shots. She said Sandoval fell and did not move. The girl seemed fine, the woman said, then moved to the grass in front of her home and collapsed.

Jenkins said, "This street used to be one of the best; now it's one of the worst. This neighborhood really went to the dogs." He said that someone shot a hole through his SUV's window recently and that a car nearby was burned. Gang graffiti appeared only once, he said, and was quickly cleaned up.

Capt. Darryl Smith of the Herndon police said Sandoval's shooting was indicative of the gang problems across Northern Virginia. But he said there had been some improvement in Herndon, especially after a regional gang task force was established and based there.

"At one time in Herndon," Smith said, "you could easily see gang members on the street showing signs and showing tattoos to police officers. We haven't seen a lot of that in the last year."

Fairfax police increased patrols and added tactical officers to neighborhoods with known gang presences, and the Fairfax and regional gang units monitored weekend carnivals. In their recent session, Virginia legislators increased the penalty for gang recruitment, enabled gang leaders to be prosecuted for racketeering and included machetes in the definition of a concealed weapon.

Sandoval's family could not be located for comment yesterday.

Janice Leslie, principal of Herndon High School, said that a moment of silence was held yesterday morning for Sandoval and that extra counselors came to the school to help students.

The homicide "is not a school issue. We will try to be supportive and helpful with people who are grieving, but it's not a school issue. . . . This happened in the community," said Leslie, who has been principal of the school for six years. "The only connection is that this young man attended the high school."

Though Sandoval was a freshman, students in all the grades were very distraught, said Sonya Rymaruk, a junior and vice president-elect of the school's student government. She said an announcement was made in the morning to the school describing what happened. Extra police officers and security guards also monitored the school, and many students could be seen crying, she said.

"Even though I didn't know him personally, we're all affected by it. You could tell on [the students'] faces," Rymaruk said. "We'd always hoped that Herndon was safe. This really hits close to home. It's so violent and disgusting."

Staff writer Ian Shapira and staff researcher Bobbye Pratt contributed to this report.